My 2003 Tahoe had a brake pedal squeak when you LET OFF of the pedal. Dealer said it was the boost - he did not have a stock unit so he installed a new unit from Advance. Squeak stopped but that unit transmitted a ton of feedback ( shaking ) thru the pedal.
Their "wizard" of a mechanic said that it would do that it you didn't turn the A/C on. Yeah, sure, in July or August in South Carolina - not unless you have a death wish - 104 degrees and 99 % humidity. Made him find a new GM one and swapped it out. Squeak stopped for about a month, then came back. Spent $320 in total - it's still squeaking.