Automatic Trans
~113000 miles

I just noticed a rattle at speeds above about 30 mph and 2k RPM.
- From a start, I do not hear the noise, even when above 2k until I get to about 30 mph.
- In neutral, I can rev the engine with no rattle.
- Checked the trans fluid - very high, above the cross hatched area - read low levels might cause a rattle, but would high levels?
- Replaced the heads about 10k miles ago - overheated and cracked... that issue seems to be fixed now, nice stable temps.

Going to take it to the mechanic tomorrow, what might I expect?

 

Welcome to FTE.
Lots of unknowns, so begin with the basics. If it has an automatic tranny, get the fluid within the hash marks after the fluid is at operating temp. Over filling can cause aeration of the fluid & the tranny sure won't like that.
We can insert a piece of tubing down the dipstick tube & use a hand pump to extract fluid to get the level right.
Check the fluid level after a 20 mile drive to warm it up, at idle, on level ground, after shifting into All gears including "R", ending in "N" or "P".
Mercon V is the specified fluid & that makes a difference. Do that before taking it in & let us know how it goes.