1989 5.7 Cobra worbling noise below 900 rpm

[Donzi GT250]

My 1989 5.7 Cobra engine is making as best as I can discribe a worbling noise not really a whine but like i said a worbling noise from the front of the engine. Above 900 rpm the noise goes away.
The PS pump is full, the fuel pump was new last fall, the alt is charging good and I don't see the engine water pump leaking.
As I remember I have always have had this noise to some extent but today I was running the engine without the engine cover on so i noticed it more than usual.
The boat runs great and smooth so I guess it might not be a big issue as like I said it only makes the noise a low rpm's
Any ideas where or what to look at for this noise?
Thanks

 

[bruceb58]

Re: 1989 5.7 Cobra worbling noise below 900 rpm

Take the belt or belts off and see if the sound is still there. You can run the engine for a little bit without risk of overheat.

Are all the pulleys totally free of rust?

 

[Donzi GT250]

Re: 1989 5.7 Cobra worbling noise below 900 rpm

Yes they are free of rust, also I may add that this noise doesn't happen when the engine is first started cold. But starts making the noise after running a few minutes.

 

[Donzi GT250]

Re: 1989 5.7 Cobra worbling noise below 900 rpm

Checked more today and it seems the noise is coming from the mechanical fuel pump.
As this fuel pump was installed last fall I figure this shouldn't be happening.
But at 750 to 800 rpm the noise stops.
Is it possible for a mechanical fuel pump to make this kind of noise?