My Room A/C window unit makes a ~ 5-10 sec. fog horn noise every time the compressor starts. Otherwise it works well. What is happening ?

Sounds like the compressor is having a hard time starting. I would say to take the unit outside and hose it off good. Let it dry and try it again.

Thanks for the response. I can handle that much. I gather that when it does that, it may just be dirty, or is there another less obvious reason for hosing it down?

Over heating caused by dirty coils is a good reason for excessive noise. Just hose the aluminum fins off on the back ove the unit and clean the filter. Give that a try. If it still makes lots of noise when starting it may need a new start capacitor. How big is this unit? Does it run on 115 volt or 220 volt?

Thanks again for the education. It's a small 7500 BTU Sears Kenmore single room unit that runs on 115 volt