Intermittent noise on 2008 F150

2008 F150 4x4 27000 mi There is an intermittent metallic /rattle sound (like marbles or a chain in a can) seems to be coming from front passenger side;sometimes on first acceleration , sometimes after turning a corner, sometimes after hitting a bump. Doesn’t last more than a few seconds Had my mechanic friend look at it on hoist and no apparent loose parts in exhaust system or breaks; 1st noticed it last August ( 7000 miles ago) but it subsides and then reappears with no distinct pattern. best guess is a bad wheel bearing but it’s never gotten any worse; other guess is transfer case/ chain problem but 4x4 use isn’t much and it goes in and out of 4x4 mode with no problems. We’re rather stumped…Anyone with same problem or different ideas ? Thanks.

Is it actually a metallic sound???

Have you noticed the sound more when useing the AC??

Somewhat metallic but not real tinny; no noise correlation with the ac; can hear with the climate system off.