Howdy all! First time poster, been lurking for months. So I bought a 1973 f100 fe390/c6 tranny 2wd a few months ago with 170,000 miles on her. Truck appears mostly stock to me, with the exception of pertronics ign/coil, column shift delete to b&m floor unit, and suspension/body lift. A/c system has been removed, along with all emissions gear other than pcv, and judging by the amount of dead end wires/connectors everywhere who knows what else. So I have been trying to figure out a quite disturbing loud sound, like a jackhammer hitting a 1/4" steel plate. It only happens when I first put it in drive and accelerate. It's not r.p.m. dependant. The tone is the same at any throttle, but the more throttle the louder it is. If I quickly let off and gently ease back into it, it never comes back until the truck sits until cold again. I have checked the engine mounts, tranny mounts, steering components, driveline, everything that is bolted and come loose has been checked/ tightened without change.. sitting behind wheel it sounds like it's coming from around steering box area, or that area of the engine/trans right around firewall or so..Truck is pretty tired, so concern is a little higher. Any ideas?

 

Checking the mounts was a good first step. The presence of a body lift is one potential clue though. Check for body and frame clearance on everything from the exhaust, to shift linkage (especially any old shift linkage left over?) and stuff like that.

Sounds like you basically did that, but maybe check again with that in mind. I think you're on the right path with your basic assumptions. Something is just getting past you for the moment.

Good luck.

Paul

 

cracked flexplate

 

Shift linkage is a cable, but old column linkage is still hanging on steering shaft resting on frame/exhaust. How did I not think of That? I will take it off right now and see what happens..
as far as flexplate, I had close suspicions. Not because I know much about anything, but because it almost sounds like a starter grinding when the motors already runnin. that same kinda metallic ringing tone that reverbs a bit, only the "drumroll" version of that sound. Have to pull engine to check that right?

 

Oh, and on a side note. Is the flexplate supposed to spin for awhile after cranking? If I crank it and it doesn't start right away, it spins for like 5-10 seconds, and have to wait for it to stop to crank again. I don't remember my last truck(20 years ago) doing that. Or any other vehicle for that matter

 

Fan hitting trans cooler lines. Or the fan shroud. Check you motor mounts too.

 

Fan shoud was removed by p.o. probably due to body lift and not wanting to deal with it. The sound made me think of the fan hitting something, but it's rock solid, and nothing closer than 4 inches from it. But I will check the blades and everything around it now since I didn't yet

 

its gotta be the fan if it keeps going.

 

There is no way for the flex plate to keep spinning after the motor stops. If it did the truck would not move as the converter would not be spinning for one. And how would the motor start as I think the starter hits the teeth on the flex plate no? Even if the teeth are on the converter the converter bolts to the flex so could not turn over the motor to start it.

As for the spinning noise look at the starter bendex being weak and the motor kicks it back in and you hear it spinning down.

As for the first noise you posted about if it was a cracked flexed plate the noise speed would go up & down with motor RPM. But to check you could pull either motor or trany to check.
I have had a converter come loose, factory did not tighten it down good, but that also changed with RPM and sounded like a rod or main bearing knock but oil psi was good so not a bearing.
I cant think of anything off the top of my head that would not speed up or down with motor RPM or truck speed doing the same.
Exh manifold leak, U joints, speedo cable or speedo head, anything moving making the noise when truck is moving, etc. all would speed up / down with RPM & MPH.

I cant see this happening and be able to hear it spin after the starter / motor stops spinning.
This is also easy to check, open the hood and look at the fan when this happens.
Then again if you don't have a clutch fan then don't waste your time as there is no way it could keep spinning with out something being broken and think that would show up first.

Keep us posted on what you find, cant wait to hear what you find.
Dave ----

 

Hey thanks for the good replies! Yeah it doesn't change speed, or tone with rpm. It's just louder, or quieter depending on how much throttle I am on while going from a dead stop. And after I drive a few blocks I can't reproduce it till the truck sits overnight. No matter how hard I step on it under load. But when it does happen, it's loud. Like everyone stop what yer doing, and w.t.f. is that noise coming. Mabey something just froze up in the morning? The past 2 weeks it's been 14°-25° at 7 in the morning when I go to work, and everything is covered in ice. I let the engine warm for 15 minutes everyday, but do you think mabey axles, or some kind of bearings etc, could be frozen and breaking loose or something? Clueless.

 

You may have a starter/bendix problem. It's what you hear when it doesn't start. flex plate should be bolted to the crankshaft.

 

Well I just checked your first post and being a 4x2 you don't have a front axle with ujoints or front drive shaft with the same so not that.

But it does make me think could be something with the front suspension. Does the noise happen in the same place each time? Maybe little dips or bumps or wash board?
When was the last time the front end was checked? Look for loose / frozen / broken shock. Coil spring(s) top or bottom clamps loose. Pivot bushings / bolt(s) frozen on the twin beams. The radius arms loose at the beams, they also hold the bottom of the coil spring in place. Radius arm to frame bushings bad.
Last would be frozen kingpins. My 81 F100 had frozen kingpins when I bought it. Every time you turned the wheel it made a real bad noise, kind of like a jack hammer, besides it did not want to return to center.
They are the only things I can think of that would not speed up / down with RPM or MPH at this time.

That is what I posted above about the spinning noise and almost sure it is. New starter should fix it.
Dave ----

 

On the jack hammer noise,
Next time before starting or moving the truck try and move the front of the truck up and down, stand on bumper maybe?
Do the same out back too just to make sure it is not coming from the rear.
Once started but not moving yet turn the wheel left& right to see if it makes noise.
Dave ----

 

Dual front suspension is solid from what I am seeing. Should I jack it up and check, while there's no load on it? About the kingpin. It happens when I back out the driveway, put it in drive, and accelerate with a slight left turn to head down street, and definitely coming from that area on drivers side. I think I'm just gonna tear the front apart this weekend and give everything a good inspection. Only have a few weeks before the zero degree days come, and my workshop/backyard ain't no fun

 

Squirrel or midget with a hammer? Ya never know.