dull knocking noise at hot idle?
 

1980 930 turbo
I have a dull knocking noise at hot idle is this normal? Is this the air pump? It does not do this when first started. Oil level is on the money and looks fairly new. Car runs great too. If any1 has any answers let me know.
Pete

How is your oil-pressure?

Can you locate an general area for the noise?

i want to say the left bank where the air pump is ,if the belt is to loose will it make this noise?

acually i here it thru the exhaust on the left and right banks is this just the valves opening and closing ? can i go by the druck pressure?

the valve train is noisier on the left side, cna i get away with a volve adjustment ,im wondering if its the type of muffler i have thats making this noise.

Devils advocate:
*Why would a muffler be temp dependent?
*Most valve train noise isn't described as dull knocking, but more ticking unless you have somthing really loose (read maybe broke).
*Is it rpm related?
*Does it go away as the oil pressure builds?
*Do you have a mechanics stethoscope? Sometimes a wood dowel works to narrow source of noise.
*Why not just remove belt to air pump and see if noise changes

Good luck and we all know what we don't want it to be.

i tried the stethoscope trick and cannot isolate it. it is not a metalic noise but the left bank is louder than the right .i can only hear it right out of the tail pipe. it sounds like if you had the the head off and were opening and closing the valve manualy ,its a real hollow sound.i talked to the original owner and its due for an adjustment. ill start there.

Had the same sounding noise on my 930. Sounded good on cold startup. Then after running for several minutes under load andf then back to idle, the noise was there. Diagnosis: clogged spray bar on cylinder #6. Dry rockers will knock instead of click when there's no oil.
Another area to look at is the alternator. They will actually knock a little when the bearings go on them.

Good luck

if i do find a clogged spraybar on the left bank how do i clean it out?

you'll have to inspect the lobes of the cam to see if that is the case. Remove both the valve covers and use a small mirror to inspect the lobes. Naturally, the lobes will show the greatest damage where the loading on the lobes is the greatest.
To clean the spray bars, you must remove the engine to to get to the cam towers. They come out from the rear of the engine(towards the front of the car).
If the noise is coming from the alternator, drive the car for awhile, pull the fan belt, start the engine and see if the noise has disappeared.

got ya , ill try the alt trick first
thank you

Same noise for me as well, clogged spay bar #6 and #5, cam ended up looking like worm wood. I'm just finishing up my rebuild becuase of this, in fact I removed the old spary bars last night, and let me tell you that was not fun.