Metal On Metal Squeal on 300e

Hi,

I have a 1990 300e that has an intermittent problem.
From time to time, I get a metal-on-metal squeal from the left side fo the car. It goes away immediately when I tap the brakes.
Sometimes it returns immediately and at other times, it takes a few minutes. Any ideas?

Also, the car idles roughly. I just had the cap, rotor and wires done with no change. The mechanic tells me it's not the idle control valve ( I have one ) but that it might be injectors. That sounds expensive. Is there a good way to clean them?

I live in CA and use Chevron gas and put Techron in from time to time.

Thanks,

Richard