A few weeks ago, used the air compressor to blow out the inground sprinkler system for the winter. Have been doing this for years. Since that day, every time we use the first floor bathroom-sink or toilet water- the pipes make a humming noise. Never happened before. Is this an air lock or some other problem? Suggestions?
Thanks.

My guess would be that you blew some debris from the sprinkler system back into the house plumbing. That debris would now be lodged at a valve some where. Pipes don't get stropped up only valves.
If the humming occurs only when you use the bath sink or toilet that means that the debris would have to be in the line to the bath. Since it occurs when using either the sink or the toilet (probably also occurs when using the tub) that eliminates the stop valves for the sink and the toilet.

Meaning that there must be a stop valve on the line to the bathroom. Find the valve, turn off the water and drain. Remove the stem of the stop valve and inspect. Hold a cloth over the valve and have an assistant turn on the water briefly to flush out the incoming pipe.
Note that a loose washer can also cause humming. Make sure that the washer is tight.