Our room air conditioner has a sort of sweet/sour smell coming out of it - it's not an old machine. We had an A/c man come a couple of years ago and he sprayed some gook on the coils in front - he said the stuff was not available retail - charged a nice fee- and it sort of helped, but not completely and now it's back. Any suggestions would be appreciated.

Does it have a removable filter in the front, usually just inside the cover?

If so, most do, I would try washing the filter, and spraying it with an odor removal product such as freebreeze, or Oust.

If that doesn't help, perhaps replacing that filter would be a good thing too, and not very expensive.

You have the old dirty socks syndrome. Clean the coil with a bacteria killing agent.