Whining, Clunking and Humming
It's difficult to nail down exactly how your car may sound if there's transmission trouble, but one thing's pretty certain, you'll probably get a that-doesn't-sound-right feeling when you hear it. Every car is built differently, so the sounds they produce can vary greatly, but if you have an automatic transmission, there's a good chance you may hear a whining, humming or even a slight buzzing sound [source: Lee Myles Transmissions and Auto Care].
With manual transmissions, the sounds will usually come across as a bit more abrupt and mechanical sounding. If you shift gears and hear a clunking sound, then you definitely need to have it checked out by a professional [source: AAMCO]. But a clunking sound from underneath your vehicle may not always point to a transmission problem. Your constant velocity joints (CV joints), or even your differential may be the culprit [source: AAMCO].
The sounds you hear may happen from time to time at first, but if you neglect the noises, they'll occur more frequently as time goes on.