My 2007 RX 350 sometimes makes a knocking sound when it's going higher than 60mph. Anyone know what this knocking noise is coming from?

 

So I take it you already checked the engine oil? What kind of knocking noise? Shallow or deep? How many miles? What has been do to the suspension? Or is it original? Really hard to diagnose vehicle noises w/o seeing it Weird think is you said sometimes, so is this under a load or what? Bumpy roads, ect? I'd figure out the engine oil pressure via a mechanical gauge first. Then have a look at the suspension......Really best to take it to your mechanic/shop as they can diagnose it best.

 

Another common problem is the front of the plastic gas tank cover coming lose. It makes a knocking sound similar to a suspension knock, but you will get it at speed without going over bumps.

 

Wheel bearings would be my guess- if it progressively gets faster and then slower to no sound - that’s it- especially if over 50000

 

Very well could be, but that'd sound like a shallow grinding noise, getting louder as speed & time....The OP mentioned a knocking noise.

 

This is a tough one without some more clues.
But since you said "knock" instead of "thump," I'd guess it could be strut-related, or maybe a wheel bearing as suggested above.
It could be lots of things. It could be a loose or broken or [missing rubber grommet] exhaust hanger. Motor mount, maybe.

 

Really best for the OP to take your vehicle to a shop.

 

^^^This.
If you lay down under the driver's side just inside the rear tire you'll see the plastic tank guard and feel if it's top is loose and hitting the subfloor above. Mine was (an 07). And I could see where the factory installed a foam sticky tape to keep it attached or quiet. Anyway, put some new foam tape in there and it will stop making noise.

 

plastic tank guard. it's top is loose and hitting the subfloor above. This was on mine too. Solved the problem with stuffing a piece of hard foam in there to hold it still.