Anything besides diffs make a cracking sound?

[ICUBoy]

Thanks to all on this board who have inspired and problem-solved for me so much.
There was this noise from the front of my truck on hard starts and stops.
I have shimmed the front diff in my 3906 (emaxx with Feigao 8xl and mm) over and over (thanks to lincpimp and NitroRCracer elsewhere for the info). I thought I did a good job, with a nice smooth diff action when turned by hand and very litle play / no binding.
There is a persistent "crack" sound from the front of the truck and now I am starting to wonder if something else might be making the noise.
Any ideas anyone? Thanks very much.