I tried asking this question in the Drivetrain forum a while back, but I think I was a little too wordy in how I asked the question.

Anyway, I've got a '72 F-100. Stock D44 and 9" front and rear axles, motor is a 400, C6 trans, divorced NP205 tcase. U-joints are all brand new (all 8 of them).

Problem is, when I accelerate up to about 35 or 40 and then let off the gas to cruise, I get a pretty nasty grinding sound coming out of somewhere. I'm more inclined to think it's from the transmission, because the transfer case sits pretty far to the rear of the cab and this sounds more like it's coming from below the trans tunnel.

The grinding sounds just like you're trying to force a manual transmission in to gear without using the clutch. It only seems to happen when you're floating in that zone between 'drive' and 'coast', and -so far- doesn't seem to be causing any actual problems (I thought maybe it was a tailshaft bearing, but there's no transmission leak, no apparent shuddering in the drivetrain).

Anyone think of what this could be off the top of their heads? I'm really hoping it's the transmission rather than the transfer case, because I've got another C6 kicking around. Oh, I should mention that it happens in 2 hi and 4 hi (haven't tried it in 4lo), makes no difference.

Thanks in advance.

 

Start with grease zerks. Fill them up completely! And then some. Get all of the old grease out of the rod ends.

Then I'd look for loose bolts. I used to hear crunch sounds when I had a bolt loose that connected the firewall to the tub.

 

Does it grind when coasting and then jerk a little when you give it some gas? (not lightly, but not to the floor either, just moderate acceleration after coasting)

What about your hubs? Needle bearings? I doubt it is either of those since you say it sounds like it is in the transmission tunnel, but driveline noises travel well... its suprising.

 

Driveline noise is a pain to isolate. You can have stuff in the rear that sounds and feels like the front.

You may want to check the slip yokes, if you have them. They tend to cause some noise too if they are dry. Lube up the shafts and see if that helps. After that, I would be more concerned with differentials, that's were you get some noise also from drive to coast. May be low on fluid, may have a ring gear bolt loose.

Like above, drive line noise is hard to pin point.

 

If it were his slip yokes, wouldnt that cause vibration more regularly, not just when coasting?

Not to mention binding when his suspension tried to articulate... Maybe there wouldnt be enough resistance to create binding.

Anyways, like the other guys said... just lube everything you can and check fluid levels. If that doesnt fix things, then start trying to isolate it.

 

Not necessarily. They tend to clunk when going on and off the gas as the splines hit against each other it they have no grease.

 

Oh ok. Never had that as a problem personally... therefore i wouldnt know.

 

I hadn't thought about it, but my front driveshaft slip yoke is pretty worn, and since I plan on replacing it, didn't really worry about greasing it. I'll go over the front end with a grease gun and see if that cures it. I did grease all of the ujoints, along with the rear shaft slip yoke.

It happens right in that little zone between very light throttle and no throttle (like when you're trying to maintain speed from around 35 or 40 up to 55 or so).

Other than the sound, I really don't notice any other issues. If you couldn't hear it, you wouldn't think there was any kind of problem. I know it isn't my hubs, because I drove for a week before I put the front axle shafts in (waiting for diff parts) and it was exactly the same.

Thanks for the suggestions, though, guys. It's not going to be back on the road until the new steering box gets here, but while I'm waiting, I'll hit all the zerks with some grease and we'll see what happens. Any other suggestions are welcome.

 

I had it bad with my, uhhhh chebbie, it had the open slips and tended to dry out every 10K or so. Drove me friggin nuts trying to find the noise. Every time I would go from gas to coast, and especially from a stop, it sounded like something was gonna fall out. Then again, maybe something was. Hehehe!

 

Is your truck lifted? Mine did it from the angle of the front shaft on my old truck.....

 

It's about 4" over stock, but the transfer case is not original, and was put in a little bit low to keep the ujoint angles in check.

No kidding. When I first drove the truck down to get gas after getting it on the road, it was the first time I'd had it over about 20 mph. My steering box had about 180 degrees of play in it, the brake booster was doing nothing, and when I let off the gas it sounded like the tranny was preparing to "jump ship" so to speak. Scariest drive I've had in a while.

 

Just a thought have you checked your rear diff? If your pinion has backed off you'll get similar noises when coasting or at steady no load speeds.

Pat

 

That is a thought, and I will check it, but I just had the 3rd member rebuilt with brand new r&p and spiders/side gears, and a new yoke. The shop I took it to seemed pretty competent (couple of good ol' boys in Glide, OR, do a lot of bigrig work), but sheet happens, so I will check it out.

 

By chance, is there any excessive slop in your transfer case shifter(or does it shift easier than othe tc's do)? If there is it might be wanting to jump out of gear ever so slightly in the situations you describe above.

 

No, actually, the transfer case action feels really clean. What you're suggesting has kind of crossed my mind. At first I thought maybe it wasn't all the way engaged, so I put my hand on the lever and just sort of put pressure forward and back to see if it changed the noise and it didn't. Good thought though.