2000 Echo: mysterious noise in heater

This winter, my little green Echo sprang a noise. I noticed that when I turned on the heater plus fan that it would often (but not always) make a ch, ch, ch, ch, ch, ch sound that would last maybe 3 seconds then fade away, only to be reawakened when I started accelerating after a traffic light. For a while this winter it seemed to happen multiple times in a jaunt about town. (I would usually get a bit nervous, promise myself to have it looked at by my Echo car-diologist, and then turn off the fan for a while.) Just when I was about to take it in, it stopped happening, even with the heat/fan on full. And now it has started happening again. I know peanuts about cars, but my intuition says it’s some kind of belt in the fan that’s slipping. Does someone with actual knowledge have an idea about what could be going on? Thanks! (The car has about 75,000 miles on it and I live in Chicago, that rattling town).

There is no fan belt for the blower motor. It is direct-drive. IMHO, one of three things are possible:

-Some foreign object, such as a leaf or twig, has worked its way down into the blower housing from the outside air inlet and periodically makes contact with the squirrel-cage fan wheel. When you accelerate, the G-forces move the object just enough to make contact with the wheel briefly, and the rotation of the wheel again knocks it out of the way.

-Your blower motor bearings could be starting to fail, so that there is enough movement or “play” in the motor shaft that the fan periodically hits the housing until centrifugal (or centripetal if you’re a physics nazi) force stabilizes it when it gets up to speed, and it no longer makes contact.

-What you’re hearing may not be the blower motor at all, but instead some vacuum-operated actuator responsible for moving the ‘blend door’ or another component. Periodically the climate control system commands it to make an adjustment, or you may have a slight vacuum leak so that when engine vacuum drops on acceleration (which is normal), something (such as the blend door) moves and the actuator needs to ‘push it back’ into the normal position when vacuum “pressure” is restored.

Unless the sound worsens or starts happening all the time, I’d probably just ignore it for now.

Thanks.so much, Oblivion!