My truck was running hot, then broke down on the highway, and it was totally dead... The mechanic put in a new distributor and a new thermostat and now it runs perfectly fine, lots more power and my gas mileage seems to have gone up quite a bit, steady idle. However... there is a rattle coming from the engine, it sounds like the valve train chattering for some reason. The weird thing is, when I'm moving and I accelerate is when I hear the rattle. parked in neutral revving it, I don't hear it. In park, revving it, I don't hear it. My brakes are metal on metal, maybe could that be it?

 

It's not going to be brakes but it sounds like it's only happening with a load on the engine. Possibly you've broken the flywheel? You can put it in drive and rev it up (powerbrake it) and see if it makes the same noise and that might lead you in the right direction.

 

Detonation? That can happen under load but not when free revving in neutral/park. Maybe the new distributor isn't timed correctly.

 

What would detonation sound like when driving?

 

It sounds to me like you may have jumped a tooth on timing or so, and you're getting preignition (or detonation). I had similar noises from my truck when the following things happened:

• Timing belt cover rubbed on timing belt
• Timing belt jumped a tooth on the cam gear
• One of the intake Valves got a slight bend in it from lack of lubrication
• Rod Bearing went out

The noise you hear could be symptomatic of most anything engine related. Since it a large mass of rotating and moving objects, it's possible you have the early signs of a more serious problem down the road.

Defiantly get it checked out and fixed ASAP.