Humming from somewhere....?

Okay, I'll be the first one to say, "we can't diagnose your problem over the phone". But I've got a problem. A customer brought me his car, a 93' Honda Civic 1.6L, complaining that there was a humming coming from inside the dash area. He removed his amp wiring from the fuse holder with the car running and it arced on the fuel filter bracket. From that point on, he's had problems with loping at idle, poor performance and hesitation when accelerating. So, when I got his car, I checked everything out with a stethoscope. Turns out the humming was most audible in the intake and fuel rail area. Most notably at the pressure regulator (which turned out to be bad). However, after replacing it, the sound remains. The system holds pressure now, but still...the noise. I can't figure out exactly what it is. To tell the truth, it sounds like a gurgling. Best analogy is when your using a garden hose and step on it, the sound it makes then is what this sounds like. You can hear it from the cab, but it's not coming directly from there. I think it's the fuel lines, maybe kinked? Ideas are all I'm really looking for. At this point, I've given him 2.5 free labor hours and my cost on the regulator. But I'm gonna lose a customer and more labor hours if I can't figure this out. Any thoughts would be appreciated.