Calulus

Thought I would check with you guys first before going to a Mechanic. When it drops below 37 at night and I leave the truck outside it taps loudly in sync with the engine for the first 4-5 miles in the morning then it gets quieter and goes away. If it gets very cold under 33 the noise does not start till after the first mile or so. I have put the truck in neutral at lights ( its an auto transmission ) and reved the engine but no noise is produced, only while accelerating in D/1/2 and R. When I let off the gas or maintain speed it makes no noise. The ATF is 1 year old flushed by the dealer and is at the proper level when hot. I drive a 99 SR5 auto with a 3.4l. If any of you have experianced this or know what it is your replies would be appreciated.