This is a 2011 E350 SD passenger van, 5.4 engine, 85,000 miles. I'm hearing a single click when accelerating from a dead stop. Sounds like it's coming from down low and to the right. I can hear it better when the passenger window is down, but I can't be sure. It's just a single click, reminds me of the click an AC compressor makes when it cycles on.

There are no other symptoms accompanying this noise. Starts, runs, stops, shifts normally. The click has been there on and off for over a year, but lately its frequency and decibel level seem to be increasing. Internet searches haven't turned up anything useful. Can anyone enlighten me on what this click might be?