Pulsating sound

I have a 1996 A4 2.8 5 spd. When I am driving between 45 and 55mph with the rpms between 2300 and about 2800 I hear a pulsating sound like a low bass. It is not unbearable but is annoying. Oh and it is the worst in 4th gear. Actually I cannot hear it unless I really listen for it in any other gear but 4th. Is it my engine or tranny? is it normal. Any body have any suggestions?

proballly just you engin ideling by you gears slowing you down