2001 PT w/62k miles. This morning, driving in town at 30mph (32 degrees F) (the car was warm) we heard a tone that sounded sort of like feedback in a sound system when the gain is too high, or perhaps like a small town noon siren. It faded in & out and at times was VERY LOUD. It happened a bit later at 42 & 45 mph. Then we drove home on the highway -- nothing. Tried turning off radio & turning off the fan -- no effect. The sound was quite "pure" -- didn't sound anything like a alternator bearing, for example. I don't recall that the pitch was related to speed. It sounded like it was "coming out of the dash". Called two dealerships and they had no idea. An ideas? Tnx Jim H.
No way of knowing if this is what's going on in your case, but I had a car with a foghorn brake booster. Under specific conditions of ambient temperature, engine temperature, manifold vacuum, throttle opening, cruise control use and vehicle speed, a lot of very fleeting factors met in a particular way, and the brake booster diaphragm would resonate with a very pure sort of hooting tone. It never caused any harm and there was nothing the matter with the booster. DS
Found it! Further symptons made me suspect brakes. Sure enough, found a warm front wheel caused by a sticking caliper.
