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Radiator Intermittent Hum / Tone ('91 240) 200 1991

posted by joe_1991_240 on Wed Nov 26 20:10 CST 2014
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Hello,

I noticed today an intermittent hum or tone coming from the radiator. It is barely audible in the cabin, it almost sounds like a fuel pump would. I isolated the location by using an automotive stethoscope. When I put the metal end of the stethoscope on the top of the radiator the hum / tone is very loud and clear (it is an intermittent medium hum, not whiny, sound).

I recently drained the coolant to replace the thermostat. I reused the old coolant (filtered). I opened the reservoir cap but that didn't have any impact on the hum.

The hum / tone almost has the sound of a power steering pump when turning your steering wheel, to help you imagine it in your mind. The radiator also cools the transmission fluid (auto transmission).

Any ideas based on past experiences?

Thanks, Joe

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Radiator Intermittent Hum / Tone ('91 240) 200 1991

posted by joe_1991_240 on Thu Nov 27 21:09 CST 2014
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The hum is coming from the transmission cooler lines. I noticed the transmission fluid level was high, and corrected that by delicately draining a small stream from the main pan (it turned out to be high by 18 ounces - a bit more than 1/2 quart - checked hot per the operators manual). The hum is now almost constant when in park - when I touch the lines I can feel the high-frequency vibration. So, definitely something to do with the cooling lines, perhaps they are clogged, inhibiting the free flow of fluid and causing the hum. Maybe the radiator needs cleaned.

I wonder if the transmission senses the heat of the fluid and activates a valve to send the fluid to the radiator, or if it is sent there all of the time. The car shifts well and I haven't noticed any transmission problems while driving. I will continue my research...

(PS I drilled a hole (5/64") in my thermostat - that had no impact on the radiator hum.)

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Radiator Intermittent Hum / Tone ('91 240) 200 1991

posted by joekidd on Fri Nov 28 07:13 CST 2014
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last visit: Fri Oct 24 10:54 CST 2014

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Radiators do not make noise unless they are leaking. Trans cooling lines are always flowing, no valve opens up.Might be noise from tranny traveling up the metal lines?

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Radiator Intermittent Hum / Tone ('91 240) 200 1991

posted by Bradley on Thu Nov 27 08:10 CST 2014
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last visit: Fri Oct 24 12:56 CST 2014

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The rad is humming because it doesn't remember the words! Sorry, couldn't resist. No, I'm going with the air bubble in the system slowly bleeding itself out. I've always taken to drilling about an 1/8" bleed hole in every stat flange before I install them. Eliminates a lot of simple issues.

B

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Radiator Intermittent Hum / Tone ('91 240) 200 1991

posted by machine man on Thu Nov 27 00:32 CST 2014
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last visit: Wed Oct 22 15:40 CST 2014

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I like your idea of transmission lines.

If they vibrated, the shaking could very well transmit into the radiators cooling coil for the transmission. Does one side of the radiator hum more than the other?

You might want to look and see if you can tie the two lines to each other to help stabilize the lengths in the middle. You don't want to have them rub anywhere on a casing or within a mounting bracket as that is how leaks happen!

Peek out the supports, thump and shake them about for good measure!

Phil

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Radiator Intermittent Hum / Tone ('91 240) 200 1991

posted by derek uk on Thu Nov 27 08:07 CST 2014
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last visit: Fri Oct 24 12:33 CST 2014

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Is it revs dependant as in does the "hum" change as you rev the engine? Maybe needs a buddy to grab hold of things while you monitor the sound. Mind your fingers. I'm thinking it might be a loose fan shroud or maybe it has a crack in it causing a resonance through the rad.

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Radiator Intermittent Hum / Tone ('91 240) 200 1991

posted by Amarin on Wed Nov 26 21:28 CST 2014
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last visit: Thu Oct 23 22:42 CST 2014

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Hello,

Could it be that hum the vibrations transmitted from engine via old hardened upper radiator hose. At the same time, hardened radiator's rubber mountings if it is, could not absorb the vibration. Also hardened engine mountings could not absorb much engine vibrations transmit them to all connected items. You could replace these.

Hope this helps,
Amarin.

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Radiator Intermittent Hum / Tone ('91 240) 200 1991

posted by joe_1991_240 on Wed Nov 26 22:12 CST 2014
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Thanks for the reply. This one has new hoses... I'm beginning to think it has something to do with needing to bleed the cooling system of air. Still doing my research... the thermostat I installed didn't have an air release valve.

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