97 dakota grinding noise?

hey eveyone...ok heres a weird question...about 2 weeks ago..i noticed a grinding noise...almost like i was sharpening a blade..that kind of noise..coming from the rear penger side...i had an idea what it was..and i was right..i had that tire off about 3 weeks prior...and noticed my bearing on that side was weak..loose...so...sure enough..i took that wheel and drum off and there was metal shavings in it..and the bearing had more play in it then before...so...long story short..i got one ...that had been used...on a dakota that was impounded..with 91,000 miles..mine has roughly 136,000...so anyway..it was the WHOLE rear..the bearings in this one was perfect....inside and out...a friend of mine (a mechanic)put it on for me...saturday...and monday i thought i ehard it...so i took both wheels off...bearings are still fine..and no shavings..my casing fluid is fine..no leaks..the only thing i can notice..the drum is a little dirty...im going to clean it on saturday...but...this noise happens ONLY SOMETIMES when i brake..but usually when i hit about 40 MPH and my foot is off the gas...and is coming from the rear penger side...and no..ts not the transmission..it shifts fine..and only does it SOMETIMES..and is never constant...any ideas?...thanks and God Bless--Nick