Noise from front axle?

I have tried to check the forum already on this subject but have failed in finding the answer. when I go to start my 98 XJ in the morning or after it has sat all day there is almost like a knocking sound comeing from my front axle but after the jeep has warmed up it goes away, I took my differential cover off and changed out my gear oil and while I was doing that I inpected my gears and they are all good, and the mechanic out here on the base said he thinks it might be my pinion bearings? What do you think and what are my other options on what it might be?

ujoints or wheel bearing

Are you saying the XJ makes this noise when you start the Jeep? Not when you start driving but when you turn the key and start it? If so I doubt it's the front axle. Please clarify when the noise occurs.

Sarge