Grinding noise

I just had my c4 rebuilt with a stage 2 shift kit, transmission cooler and 2000rpm hi stall. Anyway i got the car back about a month ago and have run it in for about 1000km's. Took it back to the mechanic today for him to check over it and it's all good. So i'm driving home from work pull up at a set of lights and decide to see how it goes. It went pretty good, but then i heard some grinding noise or like metal hitting against metal. Since i got my car back from being done, occasionally when i start it up there is this same noise. When driving normally it doesn't do it, only seems to be on start up and as i found out today hard acceleration. Any idea's what it might be?
Hopefully it's nothing too serious cause i'm starting to get frustrated with problem after problem lately.

yeah i know frustration comes with these cars, just some time it gets a bit much. Oh well i'll get over it!
I took it back to the mechanic this morning and he thinks the starter motor is damaged and when i take off hard the ring gear is slipping back causing the noise. I'm guessing and hoping this makes sense cause regular driving is fine, and my auto feels really good.

I've had the fluid levels in the transmission checked and there fine, no leaks anywhere. The engine was serviced and tuned just the other day, the engine oil presure is also fine. They wouldn't have picked up on my problem when tuning it as i asked them not to drive my car hard cause the auto had just been rebuilt.