Banging noise coming through the dash

I went to a Chamio car wash and was not impressed with the job. They cleaned it a second time and when I got the car back it had the banging. I was already late for work so I just left.
I think that a disgruntled employee may have dropped a coin down a vent. The noise is intermittent and can sometimes be eliminated by changing the air location options and playing with the fan. It sounds like it is coming from behind the glove box. I was thinking about getting a magnet to put down the vents and see if I could catch something. It has been quiet for about six months and just started again! Any advice?