Knocking sound in dash. Wtf is it?

[EvanD]

-Edit, seems like Jonheld nailed it. After searching "cooling fan for radio" sure sounds like the culprit. I'll document a bit here when I have time to get into it.

So. As usual I evidently suck at search because I'm sure this has been discussed but I'm not finding it.

My 97 LX just started doing something really odd. I tried to take a video but evidently my phone doesn't want to record audio so it came out worthless. I'll keep trying or use the girlfriends phone later if needed.

When both parked at idle and driving there's a pretty reasonably loud knocking sound coming out of the center vents (best I can tell). It's consistent in pace. It's happens when the key is on ACC and when the engine is running although the pace quickens when the engine is running. It's fast enough that I can't count and check a timer at the same time so I'm guessing somewhere around 150-200 knocks per minute.

It happens all the time with the blower off but appears to either diminish as I go through the blower speeds or gets drowned out eventually but is still there.

Changing the temp slider stops it briefly but it comes back after a few seconds. None of the other buttons seem to have much of any input on the noise.

I've got a couple issues in the engine bay I need to figure out (most likely belt and maybe a pulley or two) but it sounds the same in there as it has and you can't hear the knock at all from under the hood.

Anyone know what's going on and how much of a pain it's going to be to deal with?

Thanks in advance,
Evan

 

[Rusty Marlin]

does the pace quicken with engine rpm?
what if the engine is off and just the blower is on?

 

[jonheld]

Cooling fan for the radio.

 

[EvanD]

does the pace quicken with engine rpm?
what if the engine is off and just the blower is on?

No real change in pace from revving the engine.

I did misspeak though as I sit here and try this.

It does it (but slower) when the ignition is in the "on" position. Not in the acc position. It's gets faster when the engine is turned on

 

[EvanD]

Cooling fan for the radio.

It def sounds kinda fan like.

I'm going to go try searching for that but if anyone has any insight feel free to share.

I suppose I'm mainly curious, if it is this, is it something that needs dealt with before driving much? Radio going to burn out if I don't deal with it?

I'll report back on what I find.

 

[Rusty Marlin]

There is not filter on the blower fan, might want to pull that and look; see if a mouse left you a present.
the fan will run faster with the engine running as there is higher voltage 14.+ vs 12+

 

[EvanD]

Searching with the right terms def pulled up this issue. Although I'm having a hard time figuring out how far in I'm gonna have to dive in.

Thankfully (and sadly I suppose) I've got enough work to do on her that she won't be going anywhere far for a while (not my DD).

I've never pulled the dash on this one but it's added to the list. Looks like this topic isn't covered all that well (or at least most of the pics/links are old and don't work) so I'll snap some pics and document it a bit when I get around to it.

From the looks of it it's a damn expensive OEM part (almost 300) even if I can find one so if it actually needs replaced I'll prob look into a cheaper computer fan. People have talked about it but I've yet to find first hand account from someone that made that swap.

Thanks guys!!

 

[jonheld]

-Edit, seems like Jonheld nailed it. After searching "cooling fan for radio" sure sounds like the culprit. I'll document a bit here when I have time to get into it.

A very quick diagnostic would be to pull the 10 amp GAUGE fuse by the driver's left knee. You will lose power to all your gauges, but if the noise stops, you have found the issue.

 

[EvanD]

A very quick diagnostic would be to pull the 10 amp GAUGE fuse by the driver's left knee. You will lose power to all your gauges, but if the noise stops, you have found the issue.

Thanks dude.

I know exactly where that fuse is. Mine likes to randomly blow a few times a year. Always fun losing power to the gauges while driving but it's never been a big enough issue to deal with. I just replace it when it happens.

Edit - Doh! Of course I go to try it and the noise isn't happening now. Gonna be one of those I see.

 

[YMT]

Is the radio cooling fan on the factory head unit, or is it the little fan that's mounted near the clock, in the dash? If it's the latter, is it necessary for an aftermarket radio?
I pulled out my dash a couple days ago and noticed that little fan up behind the clock, and didn't see an obvious need for it.

 

[Njck22]

I had a knock in my dash. I pulled the gauge fuse to check if that stopped it because the fan is on the that fuse It did not. It turned out to be the air mix servo motor. It was unable to complete a full cycle when I moved from hot to cold on the slider, and would get stuck going clunk clunk clunk clunk. I had to pull the entire dash to change it out but solved it.

you can diagnose it by pulling the radio and then when its clicking push on the linkage and help it complete the movement.

 

[EvanD]

I had a knock in my dash. I pulled the gauge fuse to check if that stopped it because the fan is on the that fuse It did not. It turned out to be the air mix servo motor. It was unable to complete a full cycle when I moved from hot to cold on the slider, and would get stuck going clunk clunk clunk clunk. I had to pull the entire dash to change it out but solved it.

you can diagnose it by pulling the radio and then when its clicking push on the linkage and help it complete the movement.

Thanks for this. It sure sounds like the fan but def awesome to have more to keep in mind when I get around to pulling dash. It's def gonna be a "while I'm in there" occasion.