Tell me it's something simple :/

What's up gentleman and ladies. Long story short, sold 12 mpg truck. Bought a 2005 TSX AT with 74xxx miles on it two weeks ago. Had it for a week and this rhythmic knocking sound comes and goes every second or so, and btw only in park or neutral. Completly goes away once in drive or reverse and the sound is not always there. Probably about 75% of the time it's there. The sounds seems to be coming from tranny area (but I am half deaf due to my wonderful career of running redlights with sirens and air horns . My brother in-law is actually a Honda tec for 15 years and he can't pin point were it is coming from. The car drives perfect its just the noise is annoying.

I have seen a lot of threads on here with a lot of different noises but they all seem to be occurring while the car is in drive. Please someone tell me this is a stupid, easy fix.