E-REVO POSSIBLE MOTOR problem

[hootie7159]

whats up guys... i'm just recently converted my revo 3.3 into brushless...
-mamba max esc w/ ubec and fan w/ holes in case
-feigao 540c 8xl motor with fan on top
-5s 8000 truerc lipo batt
-geared 16/40 mod1 pinions and spur (both steel)
-stock revo tranny locked in 2nd gear with center diff
-mechanical front and rear disc brakes (used on third channel)
-esc still has reverse enabled
-stock settings on mamba w/ cutoff at 15.8v

anyways, i've run my 4s 8000 in my e-revo with no problems..however, wen i hooked up my 5s, everything was going fine (5min into the bash session) until i heard a grinding sound coming from the car...i immediatley stopped and took it home to see what had happened....well, it wasn't the pinion or spur gear and i've replaced the entire tranny (old tranny was fine couldn't find any stripped gears) put it back together and still the grinding sound...then i replaced both the front and rear diff put it back together and still the grinding...so now i suspect it was the motor...the grinding only happens when a load is palced on the rear wheels and when i gun it to full throttle from a stanstill...its definitly not cogging but grinding is too loud to pinpoint exactly where the sound is coming from...let me know what you guys think...i've already orderd a neu 1515 2.5d and i'm just wondering why this happened and if this motor is salvagable...thanks oh yeah...mamba arms perfectly fine and switched the batts to 4s and my 3s and the grinding sound still comes form the motor....