As I was parking my truck after coming back from lunch today over the normal tick, tick, tick of the injectors I heard a knock,knock,knock that went away (parked) when I gave it a bit of throttle.

So, I shut the truck off, and started it back up and..... nothing for the first little bit.
Then it came right back, so I opened the hood and its almost imposable to hear from under the hood but I can hear it easily from the divers side wheel well.
It lasted for a few seconds and then, gone again.

Comes and goes, no idea what it is, any advice?

I have had the truck since 09 and it had 20K miles on it.
Changed the oil every 3K (last one was at 4K cause I didn't have the money to spare) and the injectors have always ticked, but I have never heard this noise before.
Truck has 62K on it now, mostly all highway at 55-60 with the cruse on.

Fuel filter was changed at 40K (or close I could look and see what date/miles I did it in my book).

Trans has not been touched, not had time or money but I plan on doing it this year.
Gas keeps going up and my budget gets tighter and tighter, paid $4.10 last time I filled it up.

Any advise is welcome.