There is no CEL (almost wish there was).

The vacuum lines I can easily inspect look ok, and do not seem dried out or cracked. I can just replace them anyway. I could measure the vacuum, but not really sure what to look for.

I am not familiar with the sensors on this car.

I thought of the idle air control valve, but the idle seems fine.4, sop I have not touched it. Cold idle around 1500, then slowly drops to about 500 rpm when the engine warms up. Only a little rough, but unless you were looking for a problem, you would not notice. I have not read that section of the manual yet, but at least on my other car, the IACV is completely out of the picture once you step on the gas and the throttle opens.

Whatever it is, the noise picks up as accelerate. Almost sounds like a different engine. Not simply louder, like another kind of noise added to normal engine noise. Like an engine with oil starvation, almost, but it is not rpm dependent.

Maybe engine running lean?