I've noticed for the past week that the O2 sensor seems to be noisy at idle, but only sometimes when the engine is hot, but never when the engine is cold. The noise sounds like a bad relay. At first I thought it might have been a relay under the dash, but upon further investigation, it appears to be the O2 sensor, which I can hear through the floor/firewall. Last week I installed an MSD blaster coil. Could it be causing some kind of interference? The O2 sensor (Bosch) is about 4 months old, as are the edelbrock headers, and the truck passed a smog test in January. The engine is a 305 or 350. I don't know as the previous owner had it replaced years ago, couldn't remember what was put in, and the long expired warranty card for the replacement doesn't have the engine size. Any help would be appreciated. Thanks in advance.

O2 sensor has no moving parts to make noise--check again. Is it not screwed in all the way?

The O2 sensor is tight. I installed another one I had lying around, but the noise continues, so it's not specific to the sensor. May be it's not the sensor as I first thought. In the last few weeks I've also installed Taylor wires and Bosch cap & rotor. I can't think of anything else at the back of the engine that could make that kind of noise. Can a fatigued factory oil pressure sender make any noise?

Is this a clicking noise? Oil pressure switch should not be making a noise. By your description this is coming from drivers side. Could be exhaust leak- sometimes exhaust leak will sound like noisy lifter. What about a noisy lifter/rocker arm? Also a spark escaping from the spark plug wires could do this, or a plug wire off of the plug. Good luck!