clanking sound, put into 4H goes away
 

I have a 2004 f-150. I have an intermittent cklanking sound at any speeds. When it happens I put the swith for the 4 wheel drive to the 4H position and it goes away. the nooise comes back sometimes in at varius times and speeds. If I hear the sound and I dont put the switch to 4H it will continue to clank until I come almost to a stop then it will make a large Bang sound and ite clanking will be gone when I start driving again. It always comes back though at any time. Sounds like a transfer case problem?.... Any ideas?

Thanks

RE: clanking sound, put into 4H goes away
 

sounds like a vacuum leak in the 4wd system or the 4wd vacuum swithc behind battery is bad