bang in front end in 4WD

[celts55]

When I engage the 4 wheel drive I get a banging noise that sounds like it's coming from front drivers side. It happen rendomly the while engaged, but there is no noise when in 2 wheel drive.
Any thoughs where I should start?

 

[OffTrac]

What surface is this occurring on?

 

[celts55]

right now it's mud, as it's mud season here, but it was also somewhat noticable in the snow on blacktop.

 

[Mace350]

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