hvac pipe noise, how do I stop it ?

We have a 10-ton external unit coupled to a 'blower' in a closet on the second floor. From what I can gather, the pipes feeding the indoor unit run under the floor and then run vertically to the indoor unit through a wall. Now the problem is that every time the system kicks in, you can hear a 'rushing' noise through the wall carrying the pipes. It's not constant but grows and fades every few seconds. It's loud enough that we have to adjust the TV volume regularly to compensate. I'm wondering if there's anything I can do short of filling the wallspace with foam. Do systems like this have some sort of expander valve in the inside unit that could either be cleaned/replaced ?? Any advice would be welcomed. Gregory Nicholls.
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Foam amplifies certain frequencies, I covered my exterior walls and now I can hear the nieghbors kids running upstairs. For months I thought it was an animal in the walls. Higher rushing sound frequencies may be absorbed or may not. Rock wool,- fiberglass would work.
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Gregory Nicholls wrote:

Sounds like thermal expansion valve (TXV) 'hunting', a normal part of operation.
The best thing to do is to reroute the lineset to get it away from your living space. I'm not sure that adding foam to the wall cavity will help much.
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This is Turtle.
What you have is a freon flow noise and is hard to deal with. The only thing that i can think of right now to slow it down would be make sure you have a expanion valve type metering device on the evaperator coil and then put a celinoid valve and coil in just before the expanion valve to hold a volume of liquid in the liquid line and then have the valve open and close on demand as the signal for the cooling or condenser is called to come on. This may slow the noise down a good bit.
Now is the noise coming from the tubing set in the wall as it is coming into the house or is it coming from the inside unit area / air handler / furnace area when it is making the noise ?
TURTLE
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Based on the description, I'm wondering if this is a pump down system that has a solenoid in the condensing (outdoor) unit. Moving the solenoid valve to the indoor unit might help but I don't imagine it would completely eliminate the noise. With that size of unit, it would have been a good idea to try to keep the lineset out of the wall or at least in a wall of a closet, etc. Relocation or attempts at insulation are about the only options that could solve the problem completely.
- Robert

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Firstly thanks everyone for the replies. I now know the magic words 'hunting' and TXV. The noise is definitely from the pipes and not the inside unit. The noise is worst in the downstairs room where the pipes run up inside the wall. In fact this is the only room where it's really a problem. The room immediately above this (where the pipes turn and run under the floorboards to the inside unit) is acceptable. I doubt we could relocate the pipes without causing a worse problem. If they were to run through the roof then the noise would likely shift to the bedrooms. I'm thinking that it might be something to do with the pipes being vertical at this point. With systems like this does the expansion happen in the outside or inside unit ? ie, am I hearing gas or liquid ? Thanks again, Gregory Nicholls
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I'm more curious as to if and where a solenoid valve is mounted. There are a few things to try, my comment about relocation was that it's the ONLY way you're going to get rid of ALL the noise. You could very well lower it to tolerable or barely noticeable without moving the lines.
What you are hearing is based on how the system is set up and it's impossible to see from here. Who knows, it's possible that it's low on refrigerant. You would be well served to find a quality service company to have a look to see what solution is best now that you are a bit more aware of what your problem could be.
- Robert
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