2000 Ford Explorer possible catalytic converter problem

My wife's 2000 Ford Exploder has had its check engine light come on for ~ 2 years but until recently it ran fairly well with decent power so I wasn't all that worried about it. Two error codes have repeatedly come up when I checked it with my code reader (i.E. Bank 1 too lean & Bank 2 too lean). If I erase the codes, the codes used to come back in 40 or 50 miles, but now they come on quicker and the engine has developed a problem I've seen (& heard) with other vehicles I have worked on. Specifically, when you're driving down the freeway at 50-60 mph and not giving it much gas, it does fine, but if I give it a lot of gas quickly, the engine rpms will actually decrease. If you slowly accelerate, it can increase RPMs but when you push the gas pedal down pretty far, the engine will start to decelerate. I wanted to listen to this engine symptom more closely so I popped the hood and quickly romped the accelerator cable by hand (standing in front of the vehicle) and I can easily and consistently (whether the engine is hot or cold) hear a deep WAAH noise coming from the air filter intake and you can audibly tell the rpms are actually going down instead of up. I've checked for vacuum leaks (at the advice from a Ford dealership) and replaced both O2 sensors when we first got the vehicle but that didn't get rid of the codes. It only started making the WAAH noise about 2 or 3 months ago from what my wife tells me (I don't drive it). I'm thinking the catalytic converter is at least partially clogged and won't let larger volumes of combusted gas go through and that screws up the gas:air ratio, hence the ECM/brain says both banks are too lean. The catalytic converter is under the passenger side of this vehicle so that means I can't put on a moderately priced universal catalytic converter on and she has to spend ~$250 on a new one. I'd hate for her to pay for a new one, and I install it and then find out the engine still goes WAAH when I punch it. Please advise to keep me out of the doghouse.