Growling sound while in gear Toyota Tacoma?
First thought. NOT GOOD!!
Secont thought. WHere is the growl? If from the engine then get it to a shop ASAP. But check around. Could be the alternator lost a bearing. Or the power steering pump is out of fluid. or the fan is rubbing on something. Next check the transmission fluid level.
But find the source!! But you also say it happens when in gear. Manual or auto transmission? If manual does it happen when you put the clutch in and then go away? Could be the throw out bearing.
Is it only when in gear? Then it could be the gear box has lost its seal and thus lost its gear oil. You will need to go under car and check these. Get a manual because they will show pics of the actual fill holes to check for your model.
Automatic transmission making the sound. Check fluid of course but after that you will need to take it to a repair shop. Your torque converter could be bad or the internal pump. If its growling that usually is something rubbing. Thus metal pieces are everywhere inside and it requires a full flushing and replacement of parts. Yep very spendy. I suggest at that point you locate another transmission and just replace it.