Loud vibrating-type noise on decel

[81 ZXT]

81 280ZX w/A/T - I drive about 22 miles to work. In the first 5 or 6 miles when i decelerate I get a loud noise. At first I thought it was something loose in the exhaust but now I am wondering if it could be u-joints or something. After I drive it more than 6 miles the noise is gone. Could something be warming up and only makes noise when cold? It sounds kinda like a semi useing jake brakes. Any suggestions?

[five&dime]

I would also assume U-joints but not if it goes away after 6 miles. Odd.

[MikeW]

I would try to isolate the problem some more. You say it only happens on deceleration. Is it when you take your foot off the gas, manually downshift, or brake? Is it a single loud clunk or a continuous sound? Can you reproduce it in each forward gear?

[81 ZXT]

Thanks for replying guys. 280ZX with Automatice Transmission. It happens when I take my foot of the gas. It sounds a lot like a guard on the exhaust is rattling. It is not a clunk, its more of a brdddddddddd. I am still not ruling out exhaust due to the fact that it disappears after full warmup. Just wondered if a u-joint can make that kind of a sound.

[280zx@541]

Mine does a similar sound sure if it's the same exact noise but mines the cadilistic converder, I'm going to just get a full on straight exhaust.

[81 ZXT]

Last night I jacked it up and tightened every bolt and clamp I could find. Nothing was very loose. Test drove and no improvement. It is for sure speed related and is really loud decelerating right at 2900rpm. It vibrates at other speeds but 2900 is the majic number.

[sblake01]

Take a rubber mallet and tap on the catalytic converter. If you hear rattling inside, the 'honeycomb' has become britle and fallen apart and that's where the noise is comming from. If that's the case, not only do you need to replace the cat but the muffler also because particles/pieces from that damaged cat will get lodged in the muffler.

[81 ZXT]

I will try that. That would make a lot of sense because the car sat for 4 years and now I have been driving it regularly. Brand new muffler, less than 2 months old.

[81 ZXT]

Not catalytic converter. The noise has gotten now to the point that it does it at lower speeds. Makes it a little easier to diagnose. I believe it is drivetrain related. At low speeds (less than 20 mph) it sounds like a guard hitting the u-joint of the driveshaft and is worse while turning right. Still only makes noise on decel. It might make some noise while accelerating but is worse when decelerating. I was going to look at it last night but we had storms on Wednesday and I had to clean up some downed trees and such. Maybe tonight.

[81 ZXT]

Figured it out. There is a heat sheild between the C. converter and the driveshaft. One of the weights on the driveshaft was hitting the guard. When I tightened the guard it only moved it closer to the driveshaft. Some creative bending and no more noise.

[five&dime]

Hooray!!! Nothing better than an easy fix to an annoying rattle!!!