I have noticed my XJ 4.0 (or tranny) makes a ticking/clacking noise only when on the gas and also only when in drive. Hitting and holding the gas in Park gives no sound whatsoever.

When just tapping the gas to cruise at 30MPH (roughly 1400-1500RPM) it will sound like a simple valve tick, but the more gas I give, the louder it gets, almost turns into a clack of sorts, I let off the gas it goes away completely, and when I tap the gas again to cruise, the valve tick sound occurs.

What confuses me is to why it's A) only when pressing the gas, and B) why it only occurs when in drive.

 

U-joint possibly? Mine has a similar problem but it's more of a knock when I press the gas a cruising speed. It only happens once though right when I hit the gas. I was told it was slack in my t-case. Haven't investigated any further.