I recently got P0420 code on 02 LS430 due to a leak near left O2 sensor right where Y pipe starts. The dealer wanted $3300 to replace cats and entire exhaust. Took it to a local muffler shop which cut and welded a 4 inch pipe right where left O2 sensor is located. Didn't get the code after that. But few days later started noticing high pitch humming noise which was faint in the beginning but with time got louder. Initially thought it might be due to old tires. Last week got new tires and now I can hear the whining noise more clearly now. It is only there during acceleration and when the rpm goes over 2000. The noise goes away or is hardly noticeable as soon as I remove my foot from gas. The noise is there even in cruise control. Feels like an engine struggling when going uphill except it feels high pitch and doesn't feel like a smooth engine sound. Besides noise the car drives fine.
Took it to back to the muffler shop mechanic but he dismissed it saying that it is just engine sound which can be expected in an old car with 150k miles. It is not very loud but loud enough and high pitch to be really annoying on highway to give a headache. Took it to two other mechanics who could hear the noise but couldn't exactly pin point what was causing it. I feel it has something to do with the welding of new part in exhaust since this noise was not there earlier. Can anyone who has experienced similar issue suggest what it might be ? Should replace the Y pipe with an aftermarket or replace Y pipe and muffler ? It is hard to live with that whining noise. Any suggestions will be highly appreciated.
