Surging Vibration at Idle

[stout99]

Hey Guys. I've got an early 99 that I only drive about 10,000 miles/year now. For about a year now I've heard a vibration, humming noise at idle. It's like it surges. I felt like it was the fuel pump sucking air so I planned to do the hutch mod and the harpoon mod while I was at it. Well, I did the mods this week and I have about 150 miles on it now and the vibration is different now I think, but it's still there. I can crawl under it and the fuel tank is vibrating. I'm glad that I did the mods. Everything is much quieter now but I need to get this fixed. I've had it 11 years now and plan to keep it another one at least. Thanks for the help in advance. Anybody know what this could be?

[stout99]

Doing it a little less today but still there. Maybe it's just the air as I thought. I'll know in a few days, I guess.