High pitched exhaust noise

[Shahman]

I have a 2000 4Runner Sport edition.
Installed a Magnaflow muffler, found out the O2 sensor that sits right before the Catalytic converter (under the Passenger seat side) was leaking cuz of corroded bolts. Had that replaced by a local muffler shop. Now I still hear a high pitched noise when I am accelerating and it sounds like there's still a leak somewhere in the exhaust system.
Had it checked by my mechanic, he put it up on a lift, covered the tail pipe, revved it couple of times but no smoke was coming out.

Also on cold start when i rev the engine, I hear like a whistling noise coming from the engine...

Wondering if by a long shot if anyone of you have had this sort of problem or any advice how to get this fixed.
It's driving me Insane and also think cuz of this "alleged" leak I am loosing gas way too quickly.

Thanks for you help!

 

[Brutus]

I had a whistling coming from my exhaust after I had a new muffler installed. I thought at first it was a vacuum leak but it was a missed spot in the weld.
It only happened when I got on the throttle. Even though I got under the truck and blocked off the tailpipe, it only happened when I floored it.

 

[kman55]

I second that, i would check your welds. I know when i got my new pipe on, there was a missed spot in my weld. Simply took it back up and he welded it up!

 

[HCurphy]

Troubleshooting:
First-climb around under the rig with it idleing. Feel around all the obvious spots first, where the flanges meet ( there's some spiral wound gaskets in all the flanges that wear out, MINE currently gives me a good whistle at about 2500 rpm because of these gaskets). If all the flanges look fine and you can't feel exhaust gasses escaping. Check for rusted spots all along the exhaust.
Second-If that tactile method gets you know where. Go get a sheet of printer paper and slowly move it all along the exhaust holding the paper loosely with one hand. Look for the paper to move at all from exhaust gasses escaping. If you see no movement in the paper caused by your leak, Then repeat the same procedure with the paper but move extremely SLOWLY all the way down the pipe and kind of encase the paper around the pipe but keep it off the pipe. If exhaust gasses are escaping anywhere along where you checked, It will discolor the paper from where the exhaust gasses come into contact with it.

This is how I found all THREE of my exhaust leaks.

 

[Shahman]

Thanks guys for all the suggestions... I have found and re-welded the spots I found, after the muffler installation and the O2 sensor bracket as well. (incompetent muffler shop) I have to find another one and take it to them.
Will also try the paper method near and around the exhaust pipes.
thank again!