4 wheel drive on 2001 Ford Explorer

When I engauge the 4 wheel drive I get a clunking noise every 30-45 seconds. The 4 wheel drive is engaged and working but it I feel this clunking every 30-45 seconds.

Asked by for the 2001 Ford Explorer
Sorry to see you didn't get some help with this. Were you able to figure out the problem with the transfer case?
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Have a 4 wheel drive explorer .where would I find the sencer for the 4 wheel drive