This is the noise heard with my GEN2 2006 Prius with 293K miles. I have no engine codes. However, I did pull a P0420 "catalyst" code before I began my troubleshooting. I cleared it and it has yet to reappear for 60 miles. I have replaced the Spark plugs, Coil Packs, Fuel injectors, filled up with premium fuel with seafoam, and topped off the oil with Lucus oil treatment and it made no difference. I have since reinstalled my original OEM coil packs and fuel injectors. The noise sometimes goes in and out as I attempt to observe it but it is not limited to a cold start. I can hear it when driving around town but it is too faint to hear on the highway. It does seem to speed up with higher RPM. It is obviously present while in Park and Drive but not in neutral. This video was taken while it was in maintenance mode. I have attempted to listen with a mechanics stethoscope but I could not tell where it was coming from. Other symptoms include an immediate loss of power in acceleration thus also creating low gas mileage. It can really be felt while going uphill on the interstate. I can get up to speed but it takes more power to get it there. I have let my oil run low in the past and it was 2.25 quarts low when the issue(s) started. I topped it off with Mobil 1 high mileage and then later siphoned out a quart to add the oil treatment but did not notice a difference.
