Gearbox question

[ts@n@k@s]

Hello everyone
Yesterday i took my car back from my mechanik. We did a gearbox change, because the old one didn t shift down 2nd gear.
The ''new" gearbox is almost perfect. Great shifting, stiffer than the old one, relly nice.
But...
Every time i step on the gas, a weird sound, sounds from the gearbox.
It is a sound like the one you hear when going with Reverse (iiiiiiiii something like that LOL! )
Should i worry??