I have an 09 accent with a 5spd manual at 63k miles.

I have recently started to notice a vibrating or humming noise when accelerating, but it seems that it only occurs in 4th gear. If you let off the sound significantly decreases, if not disappears. If I go into 5th gear its gone, same with 3rd. The clutch engages and disengages smoothly.

The dealership flushed and replaced the trans fluid approximately 1k miles ago. The timing belt was replaced, the coolant was replaced, accessory belts etc.

What could be causing this?

Thanks!

 

A bad fourth gear?

 

It only seems to hum whenever it is cold. Once its warm it doesnt do it.

 

Well, if only seems to hum then may be it only seems like a bad fourth gear
But, lets wait til it gets warmer before we decide what the problem really is ... hahahaha

 

Let me clarify, once it is driven for a while and has warmed up, the hum stops.

 

So, are you saying you don't have a problem??