Disclaimer: I am not a certified mechanic.
If I were in your scenario, I would probably take it back to the mechanic to have it diagnosed and fixed--potentially for free if they recognize it could have happened under their care.
As far as what the actual issues are, here are my guesses:
- Loose heatshield over the exhaust manifold
- Loose engine mount bolt
- AC lines vibrating against each other
- Timing chain, belt tensioner or both (unlikely, since your noise persists as you rev)
There could be other things that I have not listed as well. I'm guessing based off of general consensus and personal experience.
To my ear, it doesn't sound like any sort of any engine knock so that's good news. Once again, I would probably take it back to the original mechanic. If not, I would take it to another reputable mechanic. It's going to be a little tedious to self-diagnose if you don't know what to look for. I'm not saying it's impossible to though--more power to you if you do diagnose and fix it on your own
Sources:
http://www.newcelica.org/forums/showthread.php?t=289883http://www.newcelica.org/forums/showthread.php?p=826516SIDE NOTE: 45k original miles? That Celica must have been a pretty penny!
