TRF 201 Gearbox noise

[Flodjy]

Hi All,
Just getting back into racing after well over a decade and Ive just built my new TRF201.. (Well rebuilt really, because i bought it new and built.. not that i dont trust the skills of the original builder)

Ive hooked my beautifully cheap EZrun 6.5 (please dont snigger)

And the whole drivetrain seems to chirp, almost like a pulsing sound on full throttle and its really hard to determine where its coming from. I thought it might be the diff being a bit too loose, so i tightened it up a bit.. still the noise stayed.
I tightened the slipper, and it seem to lessen the noise a bit, but i cant help but feel that the drivetrain still seems really noisy.

With the motor unmeshed, the 'box feels beautifully smooth and free

Does the TRF 201 have a particularly noisy gearbox? Is there anything obvious that i should look out for when troubleshooting a gearbox on one of these things?
Maybe the EZrun is making the noise and is a bit of a pup?

Sorry for the newbie questions but previously, i sort of prided myself on my meticulous building.. any suggestions/ help would be massivley appreciated.