This only happens in the really cold temperatures I've notice.
I normally wake up, start my car to let it warm up before work. Its a 305 tbi, and i hear it on and off only when it warms up at idle and i stick it in reverse to drive away and when put into drive it goes away. After that i don't hear it for the rest of the day. It literally sounds like a bird churping, like a baby bird calling for food.. really don't know how to describe it any better.

Only happened like 7 times in a month time frame.. I think it might just be a pulley/bearing... but it sounds distinct in the rear of the engine where its the loudest..

Thanks,
Timinsk

 

Were it the front of the motor, I would definitely say either a pully bearing or maybe the water pump or alternator is going bad.

 

If its standard it could be your clutch fork vibrating..There is a thread on here somewhere about it..i found out the hard way..replaced the clutch and did not put grease on the fork ...the resulting sound is a chirp like sound..its a vibration pitch..If it is standard rest your foot lightly on the pedal if it goes away you found your issue..Not too worry this will not cause any damage its just annoying lol

 

its a 700r4. hasnt made the sound in a few days.. but it sounds louder in rear seat of engine but maybe its just echo'in and is coming from the front..