i have a 2003 cobra, had the gears change 373 and now there is a hum. the tech went back end but its still there help.

The gear change has to be absolutely to spec, otherwise - humming noise. Is it a higher-pitched hum or lower-pitched hum? Is it constant or does it change when cornering? Does it change under load? Does it disappear when depressing the clutch at speed?

Make sure diff is filled to proper level with lube.

It's a low pitch hum, and goes away with throttle off, but continueious while using throttle, I to had my gears changed and that's when it first started

Sounds like the mechanic didn't set the backlash correct.