Technical Engine Buzzing/Resonance Noise

[DaveOx]

I have the strangest noise in my '79 X1/9 (1500, w/2x36DCNF & long-tube header). When the engine is up past 3000 rpm. It sounds like a heat shield starts resonating/buzzing loudly, but coming from the top of the motor. It resonates with the speed of the engine, but the instant I get off the gas it stops completely, even before the rpm comes down. Then rev up slowly and it starts starts up and gets louder and louder. It really sounds like tappet float or something in the top end, not thunky like rod knock or piston slap. And it is smooth and quiet from idle up to just under 3K. I have removed and/or tightened literally everything possible one by one to try to isolate the problem - air cleaners, timing covers, stamped end plates and dust shields, even ran it without the belt to make sure it wasn't the water pump or alternator - but it is always still there. The motor only has 23,000 miles on it, but could have been beaten hard by the previous owner's "mechanic".

Video attached. Thanks in advance all!

 

[The Panda Nut]

No specific help, but I once had a BX that did exactly this. IT was the air filter / box lid. It was not sealing and did this. IN the end a Citroen technician at their UK HQ sorted it. he trimmed the bottom off the lid by 0.5mm and the noise went. Trimming the bottom allowed the lid to fix down tight. Add some foam or tape to anything you suspect to stop movement and see if you can identify by elimination. Frustrating"! It will be something daft.

 

[DaveOx]

No specific help, but I once had a BX that did exactly this. IT was the air filter / box lid. It was not sealing and did this. IN the end a Citroen technician at their UK HQ sorted it. he trimmed the bottom off the lid by 0.5mm and the noise went. Trimming the bottom allowed the lid to fix down tight. Add some foam or tape to anything you suspect to stop movement and see if you can identify by elimination. Frustrating"! It will be something daft.

That was literally my first thought, so I removed them completely and it is still there. Maybe the DCNFs have something loose in them or the bowls are rattling??? I guess my next step is to remove the carb tops to see if something is loose.

 

[The Panda Nut]

Check the base of the air cleaner box is not cracked too.