strange problem for the experts

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i have a built engine w/mp sc when cruising down the highway at complete throttle release my af goes down to 11-12 and i get a noise that can only be described as a dry pump running. when i goose the gas it goes away. there is absolutely no ryme or reason as to the timing of the event. i've racked my brain trying to duplicate it but can't, it has a mind of it's own. i've gone as much as 4 days without it happening but it always comes back. i'd appreciate any help you guys could give. thanks in advance

 

I don't know if its the same noise or not but my truck's rear-end had begun to hum in the exact situation that you described. It became quite annoying and more frequent. Instead of doing it on deceleration, it started doing it on slight acceleration. I never knew exactly what the problem was because it was totalled out before I could look into it more. I am sure someone else has more insight than me.

Nick

 

my buddys 99L had a similar problem. his sounded like a hoover vac at idle cut it off and the noise would fade away not sure what the problem was though it just quite one day