Recently I've noticed what sounds like almost a groan or whining sound coming from under the dash. It sounds like it could be a slow vacuum leak due to the fact it seems noticable only during higher rpm with minimal load on the engine. It does not occur when the engine is cold. It also occurs if I rev the engine up without it being in gear. It stops gradually and totally goes away if I turn the fan on by putting it on floor and the thermostat on cold. It seems to be getting a bit louder recently. I'am not sure if the heater/ac controls are vacumm operated. If so is there an easy way to examine the unit under the dash for a leak? Thanks

 

if you were losing vacuum, you're hvac would start to default to defrost.

Does the noise occur in a/c, defrost, def+floor and/or panel+floor?

 

It only sems to occur in the off position. i thought maybe the switch was bad so I pulled off the bezel trim and put my ear there but it seems to be more behing the glove box.

 

in the off position, your recirc door is closed, so if you were to lose system vacuum the door would start to open. That could be one possibility which would cause a noise. Since you're not moving, it wouldn't be a whistle caused by high air velocity @ the cowl.

If you drop down your glovebox, you can actually run your arm up and feel the recirc door assembly.

have a passenger listen over there while you rev the engine.

BTW: the mode selection is vacuum operated and the temp selection uses an electric actuator.

 

Update on the groan noise. Seems to be getting worse. I checked the door for the outside air and it is opening and clsing properly. The noise is only at higher RPM and I assume a small vacumm leak could be capable of making a fair amount of noise. My next thought is that it could be the switch. I assume the switch redirects the vacumm and I wondered if it had seals in it that may be going bad. Is replacement fairly simple and I hope not too expensive? Thanks.

 

Does it do it at a specific RPM? Try this, get a helping hand, make sure the engine is warm, try and recreate the noise, have your helper move the incoming heater hose around, in the engine compartment, its the one that is attached to the thermostat crossover. if that noise changes pitch, etc.... it may very well be the restrictor in the heater hose...mine drove me nuts for 2 months, dealership thought it was in the heater plenum. the hose is about $17 bucks, and will take about 15 minutes for you to replace.

 

Thanks for the idea. Noise is definitely RPM related and only will occur if the engine is warmed up. The heater core had entered my mind but I hadn't thought of the hose. I was assuming vacumm because it was RPM related and I figured the vacumm was higher after the engine was warm and the RPM is higher. I'll check it later this week and let you know.

 

For Shark99. I've isolated the noise as coming from the heater core housing. I moved the heater hoses around with no difference in the noise. Should I try and pinch them off and see if it stops. Which one was your problem - the one on the right or left (closest to the battey) where they enter the firewall. thanks

 

Tom-

When you say you've isolated the noise, are you in the cab, or outside the truck? I cant recall which hose was closer to the battery. The inlet hose is the one that comes off of the crossover from just above the water pump and where the tstat is located. Which I think is the right-side one, where they enter the firewall.

Its the hose that has a funny clamp that seems to be holding nothing, but when you squeze just below the clamp, the hose is hard, this is the restrictor. All I did was have the truck at the right rpm, and moved the hose around, the noise changed, but did not go away. The $17 bucks is worth at least trying it.

I too went back and forth as to where the noise was coming from, it always sounded like the heater core, but when it groans, you can kinda feel the same vibration on the hose (caution, hot!, learned that the hard way). I finally just gave up and replaced it. The service people at ford couldnt even tell me where or if I had the restrictor....or if I needed special tools to take the hose off (which you dont) its a quick release.

 

Thanks Shark99. I spotted the hose. Will change it in the next few days. Didn't know about the restrictor in line so had even considered the thermostat opening too wide. that hose makes better sense. To isolate the noise I used a stethoscope on the inside of the truck. Noise was greatest against the heatercore housing. Will let you know how it turns out.

 

My money is still on the hose....I replaced the thermostat before the hose, didnt do anything.

You will have to strrreeechh to get to the firewall fitting. Just pinch the inner retaning clip, and it should just pop right off.

If you do replace it. You will get a nasty sucking sound after you gun the throttle...this is just all the air, it will go away.

 

Tom S,

before you change the hose, why don't you put the stethoscope on it or rig up something to prevent the hose from moving and see what it does.

 

I clamped the hose and the noise went away. Released the clamp and it came back. I went out and bought a new hose. Will have to wait until this weekend when I am not using my truck all day and it has a chance to cool down. Thanks for the help. I'll let you know.

 

Tom,

FMI, what do you mean by "clamp down" ?
Prevent it from moving or prevent water pump pulses from making the hose throb?


(THis is a prime example of what NVH engineers have to do)

 

Corpsie - I used a hose clamp to stop flow through the hose. If the restrictor in the hose is not working right it may be letting too much water setting up some type of vibration or noise in the heater core. This is all theory but I hope to have my conclusion this weekend.