First, check the tension on the timing belt to be sure it is properly adjusted. If the car is equipped with an automatic transmission, changing the filter and servicing the transmission may help. This will isolate any of the items like alternator, air-conditioning compressor etc. 60K. Just today, I noticed a howling noise coming from the air intake. The noise does not occur until the engine is in the 3500-4000 RPM range.
Pitch stays constant.
If the noise begins and the gas pedal is released the engine stalls. Engine always re-starts, and keeping light pressure on the accelerator will keep engine running. Plugs and Wires have been replaced, along with air filter and a visual inspection of the air intake up to throttle body. Howl pinpointed to between air box and intake manifold. Sounds just like someone blowing across a coke bottle.