The coming-and-going at idle and going away when you rev up also are symptoms of a noisy idle speed control (ISC) valve. It's a solenoid that meters air to the plenum at idle by operating a pintle that cycles at various speeds depending on the amount of air needed. The pintle has a rubber seal on it but over time that breaks down and a sharp ticking remarkably like loose lifters is heard as the pintle slaps against the seat. Mine has gotten pretty noisy. If you put your hand on it at idle, you may feel it vibrate in sync with the ticking. On Gen2s, the ISC is found just forward of the brake master cylinder reservoir. It's a little black box with two air lines coming out of the top and an electrical connector at the bottom. I have heard of one person successfully rebuilding it with a new seal but they are not designed to be serviced. Denso probably has the worldwide monopoly on these things and it seems like all Japanese cars lose them around 100k miles.

Is that the ISC you are talking about? Or something else. I have a 3.0 Monteros, but my ISC is on my throttle body. I know the 3.5 ISC is on the throttle body too. I think you are thinking of some other solonoid thing. Should not have air lines coming out of IAC. Is that thing the solonoid to let air into and out of the charcoal canaster?