Customs Grinding Sound part throttle

Ive got a 41 Plymouth with a 350/350 combo and a Chrysler rear end 8.25. I have been fighting a weird grinding sound for several months now which is usually at part throttle OR anything past 70 mph.

So far i have rebuilt the entire rear end, new U joints, new wheel bearings etc. havent done anything with the transmission because this sound seems to come from the rear. ive had a buddy look into this rear end several times with different shims etc. he also states there are no marks in the rear end.

Its really bad once i get up to 70. after the u joints its gotten better at lwoer speeds but still there. its only under part throttle at low speeds but once i get up to 70 it sounds like absolute hell. Ive read that maybe transmission? from previous owner that was rebuilt as well. anyway anyone have some past issues with a major grinding sound at around 70.

 

Nice looking Plymouth.
SECURELY place it on a lift or on blocks and engage the wheels. Get yourself an old radiator hose and do some investigating.

 

U joints phased correctly? Check drive line angle?

 

And driveline noise can travel up and down the shaft making it harder to locate.

 

Is the pinion angle correct, relative to angle of trans output shaft?

 

Sounds like something may be grounded out to me. Exhaust hitting the frame or something, motor or trans mount. Something like that. Very seriously doubt it is the transmission.

 

yeah the driveline was redone and put on a lift to check pinion angle. we adjusted it which helped with the lower speeds but higher speeds still there. there are literally no rub marks anywhere. had several people ride with me putting their face and hands everywhere trying to figure this out.

and its not a whirrling its a straight horrible grinding sound....only reason i thought maybe transmission was because everything ele has bene checked. not saying something wasnt missed but damn i have had that rear end checked a dozen times.

 

Tires rubbing wheelwells, rear bumper scraping pavement, drag plaque dragging, exhaust hitting top of axle, parking brake cable rubbing driveshaft, flexplate rubbing smashed dust cover?

 

Have you checked to make sure that the prop shaft slip-yoke isn't bottoming out in the transmission? Remember that this will have to be checked with the rear suspension compressed to ride height or maybe a bit lower. With the rear suspension extended (no load on reear axle) it will tend to add some end play at the slip-yoke and give you a false reading.

 

That thing I awfully low.Can I assume it’s on air ride? If so, what if you raise and lower it when the problem comes in. Basically changing the pinion angle.

 

yeah so thats what i was thinking as well. which is why we adjusted the 4 link and made sure the angle was correct. it sounds exactly like a 'binding' noise...

 

yes its on bags. so it changed a little by raising it. not much. weird enough it did change more prior to ensuring the angle was correct.

 

none of that stuff on it but we did make sure the flexplate bolts were tight and not damaged. but thats the type o sound it makes as if the starter wasnt properly engaging the flexplate at high speeds.

 

the bottom is really clean under there and i canf find one rub mark....i wish it was.

 

Check your tranny mount,i had a 69 charger that did that,drove me nuts to find it,replaced the rubber mount,car drove like a new one after that.Mount looked good,but collapsed when you drove it,the noise was coming up through the entire car,was metal on metal that sounded like the car was on steel rails?. harvey

 

roger that. will do. it sounds like steel just smashing

 

Had the same issue once and it was the dust cover touching up around the crank. Horrible grinding sound.

 

one more thing. when im going slow and barely touch the accelerator the noise comes back. but i mean like a 1/4" to 1/2" of pushing the throttle. very little pressing and it sounds like its grinding. thats the only reason i thought of the plantary gears in the transmission (TH350).

 

yeah....horrible grinding. was it visible to see where it was grinding?

 

Am I correct in assuming that this is a one piece prop shaft with no center support bearing?

Adjusting the 4-link should take care of pinion angle and locating the axle properly thru it's range of travel. But even after that, any axle travel will change the effective distance between the back of the trans and the rear axle pinion flange. If the prop shaft is too long, when the rear suspension is under load it can push the slip yoke too deeply into the transmission and bottom out.

With the suspension loaded, unbolt the prop shaft at the rear axle. Then push the shaft forward and note the amount of travel before it stops. With everything assembled you probably should have 3/4"to 1" clearance in the slip yoke from its bottomed out position.

Some under-car photos of the driveline and suspension might be helpful as well.

 

With the light throttle noise, when engine torques over a little the noise starts. In addition to the trans mount I'd take a close look at the left side motor mount for movement.

 

My vote is for driveshaft hitting bottom of car, but without pictures, we're all just guessing.

Rear axle shaft bearings. Have someone follow you at night, look for sparks flying under the car. Drive shaft a tad too long.

Oh by the way, what are you referring to when you mentioned "shims in rear end"?

 

OK....3/4" to 1" going back into the tranny. i know my buddy checked all of this but ill make sure he did this too. i know what your saying though cause i told him thats what it felt like but even under no load at 70 mph the noise starts again. at lower cruising speeds its almost gone. but once that rotation gets higher its bad.

 

hahah man i dont want to do anymore to this thing....finally got AC and everythign done.

 

shims for the new gears and bearings in the rear end. im also thinking of the driveshaft too long. thats what it felt like but my buddy said it was good and he does this for a living.,..but we all know the story of 'buddy' stuff.

 

will do....had 4 people under there chcking and rechecking last week for basic stuff liek this but i will.

 

Make sure that front u joint isn’t a a bad angle to where the yoke is hitting the driveshaft. Mine will do that if aired all the way down.

 

If it was the trans, and I'm not sure it would be, the pan would be full of metal grindings. Not that much of a job to drop the pan and replace the filter and cross it off your list.

 

will do

 

i know. its the only thing i havent touched on this car....but per the original owner it was rebuilt...but we all been there before.