got only 50 miles on my new trek 4300, i think crank in creaking

Could be a loose bottom bracket or crank arm. I've had this problem before, ended up be a combo of both...

I don't think the creaking is the BB at all.

Unfortunately it is the frame. If you're the original owner, Trek will replace it free. They have lifetime warranty on frames.

Aluminum frames are troublesome around the welds, called the heat-affected zone. Trek's hardening technique is not infallible and Aluminum ALWAYS fatigue cracks, especially if you're a Clydesdale like me. I did this to a C-dale last year....chalked it up to age. Creaked for a month or so until my Trek frame engineer friend showed me where to look for the hairline cracks. Really subtle, but will catastrophically fail.

Any Aluminum frame is subject to this, even steel frames, although they are very tough compared to Aluminum, so don't be too hard on Trek.

Mike