Yeah, your track is too loose. The vibration / chattering you're feeling is your drivers slipping on the track.
At the back of your suspension under your sled, you'll see idler wheels. There'll be a bolt on each side, with a nut on it. Loosen the nut, and turn the bolts in evenly. I don't remember exactly how tight you're supposed to get it... anyone remember?
It was like 3 inches of slack in the middle of the track between the slide rails and the track at about 15 pounds of downward force applied.
The best way I've found is to suspend the sled from the rear bumper from the ceiling in the garage using a strap.
It sounds like yours is just a little loose, you could probably get away with just tightening those bolts a couple of turns each and tighten those nuts back down, and you'd be good to go.
Just make sure that you adjust both sides the same. While you have the track up run up to 15 or 20 mph for a few seconds and then recheck the distance from the slide rail to the edge of the track to make sure you have the same on both sides.
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