Unknown buzz: what is it?

Hey, my 88 bird makes a low buzzing sound after it's been running for a while, but only occasionally and only when when i'm coming to a stop from about 25 and when i'm stopped, but when i give it the slightest bit of gas, I can feel it in the pedal and it goes away. It's hard for me to explain, but i was hoping someone would know what this was and know how to fix it. It doesn't seem to affect the performance at all, but i would like to get rid of it. I appreciate all feedback.

Thanx