Buzzing noise in vicinity of ECM

I have a 2004 Tacoma, 2.4L, 5-speed manual, 2wd, 175,000 miles, that has an intermittent loud "buzzing" noise behind the passenger side dashboard. This noise started about 2 months ago, and doesn't appear to be related to engine rpm, a/c on or off, vent fan speed, engine cold or hot, etc. However, when it does occur it seems to do so more often the first 5 to 10 minutes of starting the engine; after this time period, it doesn't come back on later during the same driving cycle.
This situation hasn't affected performance, gas mileage, idling, etc. I haven't pulled any covers off of this area yet, as I didn't want to run down a bunch of rabbit holes. If anyone has had this or a similar experience, what did you find out and how did you diagnose it/fix it? Thanks.

 

Noises can be so very tricky . It happens both parked or moving?

What you hear as loud I might not even hear .Random harmonic vibration .

There are a few relays on the right side .

Might even be coming from the engine compartment or clutch release bearing or pilot .

Exhaust system?

Do other people hear the noise?

Good Luck!

 

Happens both parked and moving. Not coming from the engine compartment nor from the clutch; you can feel a vibration (when it's making the buzzing noise) by placing your hand on the front of the dashboard just above the glove box door. Not coming from the exhaust system as far as I can tell. Everyone riding in the truck can hear this noise - it is rather loud (drowns out the music if the radio is on). Thanks for input.

 

From your description, sounds like it’s loud enough… Maybe record it and let’s hear it

 

Loud enough to over power the radio!!

Something that loud sounds like a very interesting problem.

Waiting with baited breathe to hear how this plays out.

Best of luck!