i recently bought my a4 from my brother, and while driving it, noticed a noise

when in 5th and going about 65-70mph and i hit the gas for more accel...a very faint vibrating noise comes from (i think) under the hood. it's hard to pinpoint...and very hard to tell what kind of material is making the noise because it's so faint. it goes away if i release the gas or get past a certain range of RPM...or 80mph. i forgot to look at the tach

i am not sure if it only happens in 5th either, since the car is much louder in the lower gears

does anyone konw what it may be? it's a 2001 A4 1.8t w/ 43k on it. and chipped.