I posted this last week and I didn't come up with a solution yet so I 'm trying again. My truck makes a deep groaning sound at about 1500-1800 RPM in 3rd and 4th gear. It has become loud enough that it is really annoying. Has anyone experienced this before. My first thought was u-joints, but that doesn't make sense to me if the sound is at the same RPM's in different gears. Any ideas?

 

Does it only do it under load? If so, maybe ring gear. If all the time maybe side bearings on the diff. Try seeing if it goes away when you back off the accelerator or changes pitch a little.

A deep groaning to me would point me to a ring gear going bad but you didn't say where it was coming from. The pinion bearings would be a higher pitched noise usually under power and starting around 45 mph at first. If it's in the rear I'd take a good look at the ring gear and side bearings. They often make that type of noise that you describe. Just check the magnet on the fill plug first I guess.

If your truck is a 4x4 with auto hubs try backing up in a figure 8 to help them unlock then try it again. Mine are in perfect condition but the grease makes them stick a lot more when it's cold. Mine groan like crazy on the highway if they are stuck engaged.

The same RPM in different gears regardless of road speed would point me away from the rear and hubs though. Maybe a locked up fan clutch or something????

 

I meant to park it and take it up to 1500 RPM in neutral but I keep forgetting to do it once I stop.
The sound so far is always under load. I can't pinpoint the direction it's coming from. It seems to be coming from the firewall, maybe even underneath the cab.

I am going to give it a more thorough investigation this afternoon. I am driving about 30 hwy miles for work today. That should help me locate it.

Yes, it goes away completely when I back off the pedal.

My truck is 2wd.

I don't think my fan clutch is locked up because the sound goes away after about 1800 RPM

Man, you're scaring me using words like "gear" and "pinion". I would take u-joints all day long over the other things.

 

Check for your down pipe hitting the firewall. Under load the engine torque causes the engine to try and rotate on the motor mounts. This has caused the down pipe to vibrate on the firewall under load on several trucks. Just a thought. Also check your DMF if you still have one.

 

Unfortunately, I still have the stock downpipe so I don't think that's the problem. Maybe a new one for Christmas!!?

I thought automatics didn't have a DMF? Correct?

 

Good call plow. check the firewall for scuffing. The u-joints may be a cheap option to try. NAPA's p/n is 210-0178.

 

Oops, auto, sorry I missed that. Have you checked the carrier bearing on the drive shaft?

 

Thanks for the suggestions, guys. I am thinking the u-joints and carrier bearings may the the place to start, if only for maintenance reasons. At least I could rule them out and have new greasable ones. If the carrier bearing are gone, how do i check them. Would the shaft move in carrier? Still, my truck has no vibrations at 2K RPM. Wouldn't the driveshaft shake like crazy the higher the speed if the carrier bearings were bad?

I was just underneath the cab last week looking to see where a the new downpipe I've been dreaming about might go and I didn't see any evidence of scuffing on the firewall.

 

BTW the p/n I gave you is for the u-joints that don't have a carrier bearing. Sorry 'bout that. My truck doesn't have a carrier.

 

Everything vibrates at different frequencies but generally slower turning things are low frequency and fast turners are high frequency. I had to go to a two week school for vibration analysis when I was a mechanic at a chevy dealer once. It really helped a lot. I can usually pinpoint the sounds pretty fast if I hear it. Driveshafts turn fast and they do usually grind at certain speeds because at other speeds other rotating parts frequencies cancels out the vibration. If you think that is it just first look at the joints real well. If you see rust streaks spraying out of the u joint caps or an obvious loose cap then replace it. Then maybe drop the driveshaft and wiggle all the joints. If it's tight it's bad. Loose where you can actually see movement then it's bad too. They are simple.

If you don't find anything there then while the shaft is down grap the pinion flange in the back and try to move it in and out, up and down. Then with the back wheels jacked up turn it. If it feels gritty then your problem is in the rear.
Rear ends are easy. We could walk you through the set up if it comes to that.
probably 90% of the ones I've rebuilt I was able to reuse all the shims as long as they went back in the same place. Even after replacing all of the bearings.
If you don't find the noise up front though, I'd suspect the ring gear or side bearings the most since you said it was a growl. A pinion bearing is high pitched enough usually to make my fillings hurt. I hate that noise. That carrier should probably be making more like a banging noise too. Try just shifting from drive to reverse several times and have someone watch it. If it's jumping excessively then you'll know it.

On my 91 F-150 there was a check valve on the AIR system at the back of the right valve cover that caused the worst vibration I've ever had to deal with. I was still in the Navy when I got that one and it did it from day 1. The dealer kept saying it could not be duplicated after about 20 times taking it in for that. I gave up gong to them and learned to deal with it. It only did it around 60 MPH and under a steady throttle when the engine was at operating temp. I lived with it until I went to that vib analysis school and we used my truck as a guinea pig. It even took that instructor and the whole class a week to find it. In the navy I went to a similar school and was mostly using that special equipment to pinpoint vibrations that we had to eliminate to make ourselves quieter for subs. We tried my truck in class then too but we never found it.

All it was was a stupid hidden check valve that was rubbing the firewall. In that GM school we pulled the engine to find it. We only found it by seeing where the paint had rubbed off.

There was a service bulletin out for that problem a week later since that school was also the ford training center in Memphis.

 

Excellent write up. After that I believe we all can find our noises now. Reps sent.

 

m14dan, good write up, excellent info, rep points sent.

 

Thanks Dan, that helps. I remember learning about harmonic vibrations in physics enough to be dangerous. Based on what I remember, what you are saying makes sense. I ran up the engine in neutral and the sound wasn't there. I didn't hear it at all in 1st or 2nd but I did hear it again in both 3rd and 4th. It also seems related to load. I drove it for 4 hours with a good headwind last night and it was going to drive me nuts. Today, with the wind at my back the sound wasn't so loud. I'm going out to check it right now. Let you know what I find.

I like these posts. They leave behind a trail of great information for everybody else.

 

Check these pictures out. The gunk on the differential worries me.

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The last pic looks like you have a stream of grease leaking from the cup.