Noise resonance
 

I have been trying to track down a resonance on my 75 diesel and after fitting a new alternator, I used a torch to check the best when it was running.

At relatively regular intervals the belts between the crank pulley and the alternator vibrates up down like a guitar string. That is what is making the noise.

The crank pulley is a new discount rover spares item, as is the drive belt. Has anyone got any idea what could be causing this?

Did you replace the tensioner(s)?

Could be play in the water pump bearings. Whip the serpentine belt off and check all the pulleys for play and roughness.

I checked all the pulleys when I switched the belt, couldn't find any roughness or play.

Nope, the bearing is fine, it could be a weak spring but there doesn't seem to be excessive play in the belt.

Neither is the tensioner bouncing when the engine is running.

If I hadn't changed the crank pulley, this would be my best guess based on where the belt flutter is.

Do you know anyone with a T4 locally? If so worth checking the injector balance at idle and the compensation figures just to rule out a faulty injector.