Nissan Repair: High pitch sound and then thump, high pitch sound, nissan xterra

Question
I have a Nissan Xterra with approx 110,000 miles.  I recently had the brakes replaced and not sure if this is related to that repair or not, but the next day it started to make a high pitched sound when accelerating and then it made a thump like I ran over something and then the high pitched sound would stop.  We took it to 2 mechanics who can not figure out what is wrong.  This does not happen all of the time and sometimes if you put it in park and put it in 4 wheel drive and then back in two wheel drive the high pitch will stop, but not everytime.  Any ideas?  This is driving me crazy and everytime we take it somewhere to have it looked at they can not duplicate the problem.

Answer
Laura,

I would first change the fluid in the transmission and the transfer case.  From the sounds of it you have a planetary gear set going out in the transmission.  Sometimes changing the fluid will correct the noise.  This is what I think from the sounds and conditions you describe.  Email me back if you need more.  And, let me know if the fluid change works for you.