My '05 F150 (92,000 mi) 4 wheel seems like it is engaging and disengaging when activated. Makes a clunking sound on light acceleration then quits when I back off the gas. Not sure if it is engaged then or not. I noticed this on a gravel road when I had turned 4x4 on to distribute oil around the system since I had not used it in a while.

Any ideas what it might be and if it is any easy fix or a trip to the shop.

Thanks for your help.

 

Check your system for possible vacuum leaks. Also make sure that your wheel hub bearings are ok. Also inspect universal joints in axles. Any of above could cause clunking noise you mentioned. Maybe not vacuum leak , but atleast others.

 

Definitly not U joints, could be vacuum but that should cause the hubs to engage while driving which ain't happening! The clunking noise is the either the transfer case or hubs trying to engage and then back off. I 've tried in 4w Hi & Low and get the same response. I 'm thinking it could the solenoid (mine doesn't have the rain cover, we have had alot of rain in Texas here recently )

 

Solenoid! Yeah, that would be reason for that. Also check that solenoid is getting proper and good ground.