High pitched tone while engine running, idling or driving

Get a very high pitched constant tone (not a chirp) when engine running, both idling and driving. Tone will be interrupted (stop) if I touch the brake, but will resume when I release the brake. Seems to be coming from the instrument panel, left side. Can't be brake pad wear indicator as will sound when car is started but not moved. Seems to take awhile before it sounds after starting engine. Can't hear it when car is driving at highway speed, but is audible all the time at slower speeds and city driving. Started a month or so back, about the time I ran over one of those failed semi tires on the highway. The semi tire chunk really whacked the underside of my car and felt like it hit just under the driver's seat. Any thoughts?