Buzz from the center dash

[FrontRange4Runner]

Now that it's cold, I've noticed a buzz upon start-up. Seems like its coming from the inside of the center stack where the radio is located. It lasts about 5 min if I don't turn the radio on. I know that many modern units have an exhaust fan that blows away hot air, so I'm thinking it could be this. Actually, it sounds like a fan with bad bearings or that's out of balance and is hitting something. However, I don't know if the non nav units have one. This is currently my best guess because HVAC has no effect on it. Anyone know if there is an exhaust fan tucked back there somewhere? If not, any guesses on what this could be?
Observations: Turning on the radio on for short periods of time does not make it stop but leaving it on for 5+ min makes it stop (leading me to believe that heating the internal components may have an effect). The issue does not reappear after the first start-up of the day unless the vehicle has sat for sufficient time to become completely cold-soaked. The buzz is coming out of the vents, not the speakers. I've checked all of the loose things in the cabin and they are not the cause. Only seems to happen when temps are 40F or below. All thoughts would be appreciated :hungry:

 

[philsey]

Have you removed/cleaned the cabin air filter?

 

[FrontRange4Runner]

Have you removed/cleaned the cabin air filter?

Good call, I will try that and see if it helps. Thx

 

[skimr]

I have had problems with a previous vehicle where dead leaves fell into the air intake vents behind the hood at the base of the window. sucked dead leaves down into the heater system and the leaves would "flutter" when the fan was turned on. Noise came out of the vents. Didn't know what it was until they caught fire around the heater coil under the hood. Fortunately didn't burn anything up, just smoldered. Checking the vents can be a good thing.