98 S70 GLT FWD(N. America) Strange hammering air sound?

[TheSwede]

Hello everyone,
Just as I've gotten used to the creaks, squeaks, and other sounds in my Volvo, it started with a new one a couple days ago. It is a hammering air sound (like an exhaust leak) somewhere under, but nearby the throttle body. It happens intermittently at idle and is rythmic but not synonymous with engine speed. It stops abrubtly when I accelerate, but returns once the idle settles down. I thought it may have something to do with my messed up air pump, but I pulled the relay and nothing changed. It has to be some weird component because it shuts on and off even when I don't touch anything. It doesn't seem to affect the operation or performance of the car (like half the stuff under the hood) but I do have a check engine light now. This is one noise I haven't seen discussed before, does anyone have an idea what I could check? Thanks for any guidance.

Ben

[CarVolvo]

how carefully have you checked your vac lines? Sounds like you have a loose one.. and since the vacuum nipple is right in that area it could be that.

Check the elbows of the connections by pressing them and seeing if they are cracked or mushy to the touch.

If its not that could be one of the reservoirs, solenoids that are part of the system.

Also check the brake hose feed to the master.