Maxima MIL Light

My 96 Maxima is whistling. It only does it when warmed up. It was doing it only at idle and as soon as I revved a bit it was gone. Now it is less at idle, rises quickly in volume and pitch on first gas, then disappears at 15-20mph, only to return when back down to those speeds. Sometimes switching to neutral takes care of it, sometimes it doesn’t.

I checked for vacuum leaks but couldn’t find any.

I recently had my starter replaced and told the mechanic about this. He said he suspected the intake manifold gasket (he says he sprayed ether on it an it changed the engine behavior), but didn’t have time to do anything with it. As soon as I got it back the check engine light was on (it was not on before I brought it for the starter change).

The flashing codes from the check engine light are as follows:

04 01: Intake Air Sensor

03 04: Knock Sensor

01 02: Mass Air Flow Sensor

I have to look to see if I can find a leak on the intake manifold gasket, but haven’t seen it yet. I’ve been away and the car has been sitting.

Are these codes more likely to be related to a leak, or to something that was done while changing the starter and spraying the manifold?

Any thoughts? Thanks