Transmission Noise??

I had a new 454 put in my 1999 Suburban, since that was done, on start up I get a noise. When the transmission is in neutral it goes away, when in drive it starts up again. Truck shifts okay but this is really worring me. Any suggestions?, Also heard that transmission "flushes" aren't really good practice, anything on this?

Real tough to determine the source of a noise from here, you need an experienced tech to listen to this and offer an opinion. Normally full chemical trans flushes are frowned on in the industry on problem or high mileage transmisssions.