Hello everyone,

I own two wonderful Volvos, one is a 1988 Volvo 740 Turbo with 247,000 miles on it, it drives great, has great acceleration and performs like new after I have caught up on all maintenance, tweaked with it and installed some performance upgrades, it's also pretty fast. Anyway, this problem started before I replaced my heater core after it started leaking but now seems a little louder. I checked everything under there, all the vents are connected properly, the check valve for the vents under the hood was replaced, installed properly (not on backwards). The reserve tank for the vent vacuum hose is not leaking and the rubber cap is on and not cracked. I also replaced all vacuum hoses under the dash a while back, reinspected them, none seem to be leaking. The noise seems to be gradually getting worse, happens usually when boosting and it is extremely irritating, this "ckkkkkkkkshh, ckkkkkkkkkkkhhhhhhh, ckkkkkkkkkkkkkhhhhhh, ckkkkkkkshhh!" from the mid section underneath the dash.

The funny thing is that this noise seems to usually only happen when the heat/AC controls are set to the forward vents. You can usually shut the noise up by switching it over to defrost, to the floor vents, back to the forward vents, back to defrost then finally back to the forward vents. The noise will finally come back on it's own usually if left set to the forward vents.


I cannot for the life of me figure out what is causing this noise, again I've replaced the check valve I know can cause irratic vent operation, all vacuum hoses, have checked everything. Is there some sort of Servo or Vacuum Control that can cause this? I know there is some small electronic device on the bracket for the Ignition Control Module under the driver side, this also has a vacuum hose connected. Can this be malfunctioning causing this irratating noises? Is there any other vacuum control or vent control that could be causing this? Any information that could help would be greatly appreciated, thanks!