I just picked up a 1999 F150 5.4 with around 19k miles on it. Truck is in mint condition, stored in a heated garage since new. Before we made the deal I checked everything out pretty well including checking all the fluids, test drive, other usual stuff and didn't notice anything out of the ordinary. However, today I went for a short ride with the windows down I noticed a random pinging noise from under the truck. At first I thought it was a few small rocks bouncing off something metal under the truck, however it continued to do it randomly for the next 5 minutes. I would say I hear a ping every 15-20 seconds randomly. Doesn't matter if it's idling or under load, happens about the same either way. Aside from this, truck starts and rungs great and no error or warning lights have kicked on.

When I got home I started the truck back up and climbed under to see what it was. I found that the noise is coming from the exhaust system, mostly heard at the catalytic convertor. It resembles the metal expanding kind of ping, but I think its actually originating at the engine and the noise is just carrying through the exhaust system. I then went back to the engine bay and could hear the same random ping, just much quieter.

Anyone have any ideas on this? I don't know the condition of the fuel, but based on the low miles I haven't ruled out a bad tank of gas. Only other issue with the truck is there is a bit of rust and corrosion on the inside of the exhaust tips and up into the exhaust pipe. They are still shiny and silver on the outside, just a little rust buildup on the interior of the exhaust. Hoping it's just the last 1' or so that is rusted.

 

I found this video on youtube that sounds exactly like my problem. Not a continious pinging or tick, just a random ping every 10-20 seconds.

 

Exhaust system expanding and contracting as different part cool and heat at different rates.
It will stop in time.
Good luck.