High pitched whirrrrr

Just had 4wd work done on my 97 S-10 Blazer. Now from time to time the Rt front makes a high pitched whir. It only happens in 2wd and only when the gas is let off and it’s coasting. If I punch the 4wd button it instantly goes away. As long as I’m in 4wd it won’t do it. It will also usually go away if I make a full stop. If I go back to 2wd it’s usually gone for a few miles. But it soon returns. My mechanic says he can’t get it to do it but has checked bearings and other safety areas.