Noises problem
1997 Mitsubishi Eclipse 4 cyl Front Wheel Drive Automatic 198,00 miles

Car makes an almost vibrating noise when in neutral and the noise is constant. When reved it makes the noise but stops when revving stops. When jacked up, it sounds like it is coming from the oil pan, we originally thought it was the exhaust making the vibrating noise. And the noise is a vibrating noise not knocking noise. Like I stated, it only makes it constant when in neutral, when out of neutral, when accelerating. Checked oil, fine, just had a new transmission put in it in April, no oil seems to be leaking, transmission fluid fine, There is a small power steering leak around the seal, bought seal to repair. Could someone please help me with this. Single mother and can't afford to run around to different shops and am afraid for my son to continue driving it until we find out whats wrong. Don't want to do any further damage if thats the case. Thank you for any advice you may have.