Grinding noise at start up.

[diyanich]

2000 4.4 tranny rebuilt,since then weird noises are present before it gets warm.
When I first start it in the morning I get a grinding noise 2 seconds later I plug the reverse,accelerating increases the speed of it and it's gone in 15 seconds or so,but after every shift I hear the different grinding noise,harder the gas pedal,faster is the noise,no matter D S or manual shifts are engaged.it goes away in about 5 min of driving...
My tranny's got one more year of warranty on it,I am always busy to bring it down to the shop,but may be it's about a time to stop fooling around and take care of it.
Did anybody experienced something like that before?

Thanks.