Celica P0420 & Strange Noise Options

[2kCelGT]

Hi
Excuse my lack of knowledge here.
2000 celica gt 1.8 (118k mi)
TRD exhaust (or so i am told) and an intake recently added 4 months ago.
Check engine light went on the other day while on highway at 70.
P0420 (1 or both o2 sensors, CAT, or exhaust leak) code from parts store.
I think Toyota has a unit that will read more than this.

No appearant loss in performance.
Notice at 2k rpms or a little higher in 4th or 5th gear, there is an odd noise coming from the engine compartment.
Seems to only do it when there is a low torque situation with the gearing and rpms.
Was not able to repeat starting out in second gear from stop.

Hard to describe noise. It is not a knock or ping. Doesn't sound like lifter tap. Nor backfire.
Sounds more like a flutter or sputter. Almost like something rubbing against a fan blade.
Nothing in the way of the fans. Cannot hear with window open. To much wind noise.
Not sure if code related to noise.

Any advice appreciated.
Do O2 sensors operate differently at 2k rpm in second gear than 4th gear?
Does the engine adjust anything at higher speeds differently than lower speeds at the same rpms?
Could this be clutch/tranny related?

Thanks for the help.
Joe

[2kCelGT]

Update:

0420 code caused by bad Cat.

Noise unrelated.

Bearings in tranny a mess, fragments in fluid when drained.

Gears are fine. Bearings, brass, and seals being replaced.