B44 build spur/pinion noise

[carborush]

just finished building my B44
and was able to testdrive for the first time

the pinion/spur make a very loud scratching sound
as if the spur is scraping against someting (it's not, but sound like it)
when i remove the engine the chassis is rolling smoothly without sound
i fastened/loosend the central diff but this didn't make the scratching sound stop (got even louder when the central diff became to loose)
the front and rear axle diff's are very smooth and loose (mabey to loose ?)

any idea what could cause this ? any solution ? or is this normal ?

I have a 12T brushless engine with the standard 84 spur and a 22 (tamiya) pinion