[SOLVED] 2012 Mazda5 (2.5L 6MT) hissing under acceleration

[IamFODI]

Happens above 2.5k RPM or so, with heavy throttle. Most pronounced between 3k and 4k (haven't gone higher than that recently). The moment I get off the throttle, the hissing stops. Can't hear it with mild throttle at any RPM. It's pretty pronounced and has been getting worse, to the point that it sounds like a mild turbo.

Vacuum leak is a strong suspect, right? But wouldn't that affect idle quality and throttle response as well? Those seem completely fine.

Planning to poke around under the hood a bit. What should I look at/for?

Already thinking to spray some MAF cleaner or something around potential leak points. What are they?

What else should I check?

 

[Mazdiod2]

check the duct that goes from the air cleaner to the throttle body. Something might have a leak.

 

[Silentnoise713]

Intake: small bottle of propane with flex hose with bendable wire to guide hose to unreachable places, check in spurts.
Exhaust: duct tape vacuum 'air outlet' to exhaust pipe, feel/spray water for leak.

 

[IamFODI]

Seems it was the air filter.

This isn't actually our vehicle; we are just caring for it for a few months (might tell that story separately). It seems that whoever last changed the air filter hadn't properly seated everything on the side closest to the engine:

I also noticed a layer of fine dust on the inside of the clean side of the housing:

Happened to have an extra Mazda air filter waiting, so I swapped it in and made sure it was straight. No more hiss.