70 240Z, metallic clang clang clang on acceleration, worse when cold.

[Len70Zcar]

Very metallic and rapid clang/clang noise under load is coming from underneath below console maybe closer to passenger side. Goes away when you back off. Much more prevalent when cold and almost goes away when warmed up even under load. Changed rear diff mount, diff fluid and trans fluid. Only one Zcar mechanic in area and he has no clue. Last resort is to remove trans to look for a loose bolt but he's not too confident that is that problem. This is a fairly powerful 240Z with a 280 block, header, cam, flow work on E31 head. Sound just seemed to start for no apparent reason and seemed worse after mount changed and fluid changed but diminishes with driving so maybe recent colder weather is what made it worse. Any one have any ideas before I drop a bunch of money removing trans looking for a loose bolt if that even makes sense? Car pic below. not a deep sound but a lighter clang as if metal to metal but not thick heavy metal to metal. Thanks for any inputs!

[Zed Head]

The exhaust system is bolted to the transmission.  Seems possible.  "clang" is not much to work with.

There is also the bushing in the tail shaft housing.  You might grab the propeller shaft and see if it's loose.

[SteveJ]

Maybe shoot a video so we can hear the noise.

[Len70Zcar]

Thanks, I just picked it up and drove it home for an hour. The sound seems like it evolved into something more like creaking bed springs and bad spark knock singing together. It is definitely an under load condition, does not change with RPM,  and it does go away after driving 10 minutes or so. Very confusing. I have a guy that can place microphones beneath the car that connect to a tablet that may help to isolate what part of the chassis, power train, or suspension is making this racket. It is so strange that it goes away after a bit of driving. There is an occasional louder metal to metal sounding clang that happened just once at the end of the drive home. Probably two separate issues. Maybe I can get an audio file from his microphone testing.

[Mark Maras]

 You say it doesn't change with RPMs but does under load. Does it change with speed?

[Diseazd]

Crazy how it goes away. Speedometer cable possibly bad? A worn crank rear bushing can cause transmission noise.

Edited September 24, 2022 by Diseazd

[Racer X]

If it is only when the car is in motion then it has to be in the drivetrain. Transmission, driveshaft, differential, halfshafts, wheel end hubs.

Take a close look at the u-joints.

[Len70Zcar]

There was a speedo cable recently replaced/rerouted but mechanic said he disconnected it and nothing changed, They replaced Trans fluid with 75-90wt when I asked for straight 90, They then went back to straight 90 in Trans and the sound seemed to diminish to me but I need to drive e few more times to be sure. The 75/90 synthetic is still in the rear end but noise seems to be further forward than that. I'll try to get video with sound uploaded but with all the rain in coming days and having to drive a bit on an unpaved road that won't be for a week or so when it dries out. Ever heard of a bolt or nut loosening in trans that contributes to noise?   The noise is like a repetitive chatter under load sort of before it eventually stops with more driving. Hopefully my guy with microphones can narrow down the location somewhat but I won't be able to get that done for a week or two. Exhaust seems tight and mechanic further isolated it from hitting anything for test drives along with pulling shifter forward with a bungie cord and disconnecting speedo cable. Shifter used to rattle before new bushings were installed but bushing were not correct ones making it a little hard getting into reverse but that stopped the shifter chattering. Odd that this noise dissipates after driving a short distance. I'll compile all of your inputs and get them to my mechanic. Thanks for your help!

[Diseazd]

Check the u joints on the driveshaft?

[Len70Zcar]

My mechanic at the Datsun Doctor in Waterford MI doesn't seem to think it is the U joints. Believe he said that the noise from a bad U joint would not diminish after a short drive. He has worked on these cars for the 24 years that I have owned this one, perhaps longer. Unfortunately the owner and other mechanic there had to retire so down to one guy who knows these cars in SE MI, that I can find anyway. Hoping a place near home that places microphones that connect to a tablet can at least narrow it down to a specific location. He helped me find a suspension noise on another vehicle once. With the difficulties locating parts and mechanics I'll consider selling this one once repaired. Sold my mostly stock 1970 which I had for 22 years in July. Thanks for all of the suggestions, good forum! Will add a video with the sound in about a week too.

[Mark Maras]

 Have you tried putting the car on jackstands, starting it up, and putting it in gear to see if you can locate the noise while it's up in the air?

[Len70Zcar]

Mechanic said he could not hear it while up on his lift. Only under a load from acceleration then it goes away after warming up a bit, just a few miles. Hard to think of what would just go away after driving awhile. Most problems; something worn, something loose, something hitting something else under load would persist I think?