02 sensor failure though car ran ok NOW car is LOUD and vibrates at any accelration over 1500 RPM

Poor College student Purchased

a 1997 Volvo 850 Base sedan

109,061 readable odometer
(Odometer and tripometer froze)

car runs GOOD

1 WEEK LATER car was running ok accept for po410 indicator engages

PO135 Failure detected when hooked up to diagnostic machine to elctronic computer port inside console at auto parts store

2 months later

NOW since has gotten very LOUD after 1500 RPM in the exhaust area, its annoying I can feel vibration on my left side of my body from butt to head so much so bones in my skull through my left ear when driving unless a roll down both driver and passenger windows .

if kept at 1500 or less it stops

when I say nasty loud its like it has headers it doesnt have and sounds really shitty.

I couldnt afford the o2 sensor to relplace it after I just shelled out thousands for the car and let it go though have changed the fluids Oil and transmission with castrol gtx high mileage 10w-30 and used atf 3 same castrol high mileage formula group.

what is the issue and how much will this cost in replcement parts or adjustments. to car as is.