Knocking noise under the car, HELP

[Smurf510]

So a new noise started today in my 510. It only happens when I'm driving over about 20 mph. I can put the car in neutral and rev to 3-4K and no noise at all? It sounds like it is coming from the tranny area. Any suggestions on what it could be?  

[datzenmike]

 If it's in the transmission there's nothing you can do to fix till it's removed so lets concentration on things you can look at and perhaps fix.

 

'knocking sounds'...

 

Check that the engine to transmission bolts are all tight

 

Check that the transmission mounting bolts to cross member are tight

 

(while under and at the transmission check the oil level. With the bung removed on the side, you should be able to insert small finger and touch the oil) If you don't know when it was changed last change it now, GL-4 90w NOT GL-5. Must be marked 'yellow metal safe' if not sure it's GL-4.

 

Put in neutral and raise one rear wheel. Turn drive shaft and push and pull sideways (at right angles) on drive shaft at the U joints. Don't be shy, lean heavily on them they can handle it. Drive shaft should turn only, there should be ZERO sideways slack or movement.

 

Grab the exhaust pipe and pull hard on it in several directions. Does it hit anywhere? Has anything come loose inside the resonator or muffler? Is it properly supported and isolated by hangers? Has a tree branch lodged in there?

 

Are the emergency brake cables secure? Try driving along and pull the emergency brake enough to take up the slack.... does it get quiet?

 

Are the shock absorbers tight? Wheel lugs tight?

 

 

 

 

[Smurf510]

Thank you sooooo much for all of that amazing info!  I am a rookie and I appreciate the knowledge!  

[iceman510]

Possible U joint in the drive shaft (as Mike mentioned) would be my suspicion.

[banzai510(hainz)]

loose drive line bolts

 

ck all bolts

 

U u joint in drive line and half shafts