Front left wheel is making a high pitched squeeking noise. It comes intermittently, but usually when the car is warmed up. The squeeking is a pulsing sound also. You can hear it get louder if you brake, go over a bump, or turn the wheel. It's not ALWAYS there though, probably about %50 of the time, once the car has been driven for awhile. I couldn't figure out which wheel it was coming from, but I knew it was on the left side. When I got home last night, I pulled into the driveway, put the car in Park, and the car rocked a little (put in park on hill) and the squeeking would squeek with the rocking of the car. I then determined it was from the front LEFT wheel.

Then this morning I took the car to my mechanic to check it out. We drove behind a business complex (along a wall to hear the noise better), and it wouldn't make the noise! Finally, after a litte listening, it was there. But it's a really high pitched pulsating beep, and he couldn't hear it! Maybe I'm trying to be conacted by a different world... Anyway, I could hear it well, but he couldn't hear it, like his ears could not pick up that high of a noise.

Well I know this will not be easy to diagnose without seeing it. But any places I should look? I have braked hard a few times to try and remove glaze build-up, but it didn't really help. Should I put some glaze stuff on there anyway, just to eliminate some possibilities?
Thanks everyone!

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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)