Gen 2 Prius Strange repetitive noise from a/c - heater unit

it does sound like a vent door that can't decide which position to stop at. i would start with a search. you might find that someone else has already done the legwork.
if not, the service manual should cover it.

 

The two motors right by you're gas pedal foot attached to air box black square things put you're hand on to find the unit hunting . Remove look inside you'll see what to do it's indexed mark the wheel etc .

 

This thread has a video so you can compare the sound, and watch what causes it:

No, that wasn't a mouse in the heater | PriusChat

 

Are you certain its not coming from the engine compartment?

 

That is essentially the idea, but there are three different servos that are built that same way and can develop the same malfunction.

The air mix servo is one of them. The other two are the outlet select (feet / face / windshield) in the inlet select (fresh / recirculated).

The method of repair is the same for any of the three, but they are not equally easy to access. The air mix and outlet servos are on the more-accessible side of the heater box. The inlet servo is sandwiched between the heater and blower boxes and is more difficult to access.

The "no, that wasn't a mouse" thread that I linked earlier has information (if you read far enough along) about a very low-profile offset screwdriver that can be very helpful for access to some of the servo mounting screws (as, in gen 2, the brake ECU is inconveniently located and gets in the way).

If you are able, I suggest putting your head down there and watching, to try to make sure which of the three servos is misbehaving. To take them all apart and clean them all could be tempting on principle, but would be very laborious given the access issues.

 

Also if these are anything like the servos on our 1998 Accord the plastic will have become very brittle. Some of the little retaining loops that hold it together will probably break when it is opened, especially the ones near where the plug attaches. It can be reassembled without those but then the two halves of the case could separate a little there, and that would provide a path for dirt to get in. There are aftermarket units which run around $30 (see for instance RockAuto). The motors in the Chinese made ones are not supposed to be very reliable. However, the plastic case may be identical to the OEM one, so one could transfer the guts from the old one (with a now broken case) to a new (and flexible) case. The OEM motors in these were very reliable in their time, but in that Accord motor failure was observed.

The wiring connector that goes into these units may be accessible even if the unit itself is not. Unplug them one at a time and if the sound doesn't go away, look elsewhere.

A mechanics stethoscope might be helpful, but maybe less so for the the two that are right over each other, since the sound is going to transmit easily from one to the other.

 

I didn't run into any brittle-clips issues opening the servo from my 2001 in 2014.

One useful trick with that kind of enclosure held with multiple clips is to use, for example, several guitar picks or the like, inserting one at each clip to keep them all bent open just no more than necessary to get the case apart.

 

The Accord plastic was probably in better shape 11 years ago too.