Electrical buzz engine compartment. Brake related but...

Summary:
High freq noise in engine bay near fuse box for 2 days. Gone now replaced by emergency brake light always on. However...

Details.

Two days ago i got a very high frequency noise in the engine compartment near the fuse box that could be heard from the cab but for sure in the engine bay. It would go away a few seconds after turning truck off. Electrical for sure. Nothing wrong with brakes. Fluid level is OK and not the a charcoal canister vacuum leak. (I read those threads)

Could not ID the source. Listened and felt everything i could. Right side, rear of engine but deceptive.

Testing.

The only thing i could do to change the frequency was to press the brake peddle. (Done while parked so no truck motion involved) It consistently would change the frequency. Never going away but an distinct audible change. No other electrical input would make a change.

Now.

Fast forward 2 days and my emergency brake light is now on 100% of the time and the noise is gone. Cycling the E-brake has no effect.

Before i go replace that switch

I want to ask for input because that switch is inside the cab. The noise was NOT inside the cab leading me to think the problem is elsewhere and this is a symptom.

Any advise?

Images below are not mine. Happy to removed.

Where the sound was coming from.



Switch is part # 84550-34010



Location of switch inside cab.

 

Well. I feel like a dope.

Let's just say you should verify things. My master cylinder reservoir is not exactly clean any more. My quick look at it fooled me into thinking it was full.

The buzzing i heard was the fluid level sensor freaking out as the level became marginal.

That was my early warning signal (which i ignored) of my Master Cylinder blowing out. On the way home yesterday i ran critical low on brake fluid. Made it home, filled up the reservoir and promptly saw it all drain out between the MC and Booster.

New MC i guess is the order of the day.

Anyways...