Knocking under car during deceleration

[51_Meadowbrook]

My ‘51 Meadowbrook has started making a knocking sound under the car when I decelerate. It’s not all the time. It happens more randomly. It sounds like it’s in the middle of the car but it isn’t always when I’m hitting the brake. Sometimes I am just letting the car slow down on its own and it starts knocking. Where should I be looking for issues? It doesn’t sound like the clinking of a broken brake component but I haven’t taken them apart yet to rule it out. It’s not when the car is speeding up. 

[Plymouthy Adams]

maybe a squirrel built a nest under there...while he is spinning around on the driveshaft his nuts are hitting the floor pan.....uh...the nuts he gathered and stored in the nest...

[51_Meadowbrook]

Lol he’d have to have nuts of steel. I didn’t see anything just out at me when I looked under the car. 

[chuck 47 plym]

I would suggest checking the u joints. 

[Jerry Roberts]

An exhaust pipe hitting the floor ?

[maok]

Jack the car up onto 4 axle stands and try replicating off the ground.

Edited December 31, 2018 by maok
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[busycoupe]

is one of the rear engine/transmission mounts weak, or failing?  A deteriorated mount could allow the engine to move and bang against the mount on decelleration. .... Something to check...

[Phil Martin]

I put a new crank in a furd years ago. Forgot to put in pilot bearing and thats what happened.

[51_Meadowbrook]

I checked the u-joints. They are fine. The drive shaft doesn’t wiggle at all. I took the end of the exhaust and jiggled it and it moved the entire system up to the header. I also could recreate the sound of the banging. The gasket has blown out at the header pipe. So I will have to put another one in and then tighten the pipe again. 

[greg g]

Is there a sheet metal shield above the exhaust pipe where it goes under the passenger foot well?  That could be a source. Does the noise happen at a certain engine rpm while slowing. Is road speed involved, as I. It happens when you slow down to 22 mph?  Does it happen when you take your foot off the gas or is there a lag between beginning g to slow down and the start of the clunking?

[Dodgeb4ya]

It can be hard to check a U-joint with the shaft installed and loaded.

I'd remove the driveshaft... open each end up and look for gouging wear in each of the Detroit Ball and Trunnion bell housings.

Edited January 2, 2019 by Dodgeb4ya

[Jerry Roberts]

On 12/31/2018 at 11:24 AM, 51_Meadowbrook said:

I checked the u-joints. They are fine. The drive shaft doesn’t wiggle at all. I took the end of the exhaust and jiggled it and it moved the entire system up to the header. I also could recreate the sound of the banging. The gasket has blown out at the header pipe. So I will have to put another one in and then tighten the pipe again. 

It seems that he found the problem . 

[dpollo]

when deceleration is happening, and especially when the brakes are applied, the snout of the differential is forced downward enough to require the driveshaft to extend itself via the slip yoke or, in the case of ball and trunnion (sealed) joints , the trunnion balls are pulled toward the dust boot end. even  If the joints are still good, it is possible that dried or hardened grease is interfering with smooth action.   It can make a dreadful noise.

[Silverdome]

On 12/31/2018 at 1:24 PM, 51_Meadowbrook said:

I checked the u-joints. They are fine. The drive shaft doesn’t wiggle at all. I took the end of the exhaust and jiggled it and it moved the entire system up to the header. I also could recreate the sound of the banging. The gasket has blown out at the header pipe. So I will have to put another one in and then tighten the pipe again. 

Double check for broken exhaust manifold at the header while you're at it just for safeties sake.