Air bleed/leak?
 

I recently bought an 89 240 wagon (168k, 5 speed, $1500).
I checked all of the would-be issues before purchasing (to my knowledge) including the crankcase pressure to assess the PCV, which is fine. However, when driving at speed there is a noticeable "hiss" from within the dash and a minuscule amount of air issues from the center vents- sometimes hot. Any thoughts?? The air conditioning is not functioning by the way, and I'm usually running with the blower off.
Thanks

the center vents should have a pretty steady light airflow even with the fan off when you're traveling, unless the recirc button is pushed in. if you put the fan on 2 or 3, with the 3 air control buttons all out, do you get a fair amount of air from the center and side dashboard vents? after its been running a bit, is that air outside temp while you're moving (ideally do this test when its relatively cool out, like at night or early AM).

my daughter's 87 240, the heater valve failed. it had the old style thermostat valve, I replaced it with a nice all metal aftermarket valve (which is really from a old dodge truck), like this...
Volvo 240 heater valve kit replaces 1348252 9131998 $59! Volvo 240 heater valve kit replaces 1348252 9131998 $59 [] - $59.00 : EuroPartsHouse
Volvo 240 heater valve : EuroPartsHouse

if your AC is not working, you really should disconnect the control wire to the compressor solenoid so the compressor isn't running without a charge by accident. you can unplug the pressure-stat on the side of the reciever/drier, and tape it off so the two pins on the cable plug can't accidentally short something or each other, then the compressor can't run.

Yes, the blower appears to function "normally" otherwise (I've had the AC compressor inhibited as well btw)... it's good to hear something besides "you have blown every gasket there is and your engine is scrap." because that's basically what I assume every time. Sounds like I can chalk it up to normal behavior, thanks!

fwiw, I leave the fan on 1 on all my volvos (240 and 740), and normally all buttons out. I push in the defroster button when I need to defrost, and the heater/floor button when my feets are cold, and almost never use the recirc button. the 740 fan is a lot quieter than the 240, and the rotary air selection works very nicely.

The "hiss" is likely from the vent control... Probably a vacuum leak. Find the one way valve on the vacuum line going from the intake manifold that operates the vents and plug it to see if it eliminates the "hiss". Doesn't sound like you have a working vent control...