[Lewis&Clark]

Sounds kinda like leaves in the vents but the nature of the sound changes so I suspect it’s mechanical. Perhaps a fan motor starting to die?

Starts and stops as I turn on or off the climate system. If I whack the dashboard, it’ll stop for a bit and then come back on.

What might it be and what parts should I consider replacing?

[bmorey]

I've had something similar in the past, but hard to tell for sure based on your description. First thing I would make sure would be to ensure all the vents are fully open on the dash. I've had a sneaky passenger close one before and it made a weird whistling the next time I used AC. Thought I had an issue, but it was simply partially closed.

EDIT: another thing I just thought of: Check your cabin filter. maybe some debris is on that! (Its behind the glovebox)

[Humble Leader]

the 4runner blower fans are known to be a bit loud. there's always been a slight hum to mine but only noticeable if you have the radio off and windows closed (154K miles original blower fan btw). wife's RX has a hum too. many mistake this for a defective fan, even on brand new vehicles, but really they are just on the noisy side. some have replaced blower fans and claimed it has helped. if you are hearing a metal on metal grinding sound it could be the bearing going bad, at that point replace


1. take out cabin air filter. very easy on the 4runner, inspect for debris and probably replace at the same time
2. take a peek at the blower fan, look for debris. might have to remove the blower fan if there are leaves stuck at the bottom of it. not hard to do. 3 bolts. be careful not to snap the plastic trim clips on the trim below the glove box
3. if you drop the blower fan you can see if the bearing is loose, or worn out as well

1-3 above can be found on youtube, very easy DIY

[Syr4RinNC]

It might be a part called the "servo". Unfortunately you have to rip the entire dashboard apart to repair this and most people just leave it. You can temp fix it often by thumping the dashboard or cycling through the temp from high to low