mid-high speed noise

Ok, I know i shouldn't be going this fast in the winter, but at about 140-150km/h, my car starts making a weird vibrating-noise. Sounds like it's coming from somewhere around the gas pedal, but the noise continues even after I let off the gas completely, until the car slows below 130km/h.
Is there's something under the car that could cause this? (the car recently had a high-speed off-roading experience, scraped the underside.....no VISIBLE damage tho)
BTW: I have ruled out wheel problems(my first thought), and suspension.
I'm running a 94 taurus, with a newly-rebuilt 3.8l. The odometer reads 234k km


I really want to get this fixed asap, it's making me really nervous!