Ford - F150 :: 1997 - Have Thumping Noise?

I have a 1997 F150 that has been making an odd noise (something new and different). It seems to be coming from the engine-ish area. It makes a hollow whooshing kind of noise, followed by a thump/thud that I can feel through the accelerator (think frighting off beat base drum living under the hood). It sounds like a very large valve of some kind closing. It does not do this all the time, and both times it has done it have been at low altitudes- might not mean anything but my truck doesn't seem to like low altitudes- it idles low there. It has done this twice now- once 2 or 3 times in a minute for about 50 miles and then it just stopped miracle!.

I added oil in the interim of that and I thought maybe that had worked- it was embarrassingly low- took 4 quarts to get it to read half way up the dipstick. I might have murdered my truck. It did it again when I was on a long trip, coming home. It only did it 4 or 5 times. My truck kindly listened to my desperate plea and hasn't made a peep since, which means I made it home but now I can't get my mechanic to have a clue. I tried that the first time and he said "hmm." That was incredibly useful. So do I need a new truck? I might be able to afford a 1998 model.