Over the past two or three days, my HiBlow 120L has gotten noisy and as of this morning the difference is suddenly very noticeable.

I haven't been able to check it out. I have to leave for work here soon.

It's been pretty cold over the past few days with lows getting down to around 10 degrees. It's went through a much colder spell earlier in the winter and was fine. I don't know if the cold has anything to do with the noise though.

It's the first pump I've owned and I bought it and set it up last Spring. I have two lines approximately 100 ft. each running off of it. The disks are in about 4 feet of water and the line diameter is 1/2".

I don't know what is causing the noise but it's concerning. I don't know if ice buildup in the line is happening and if that would cause it. I don't know if it's the motor or the diaphragm. It still appears to be pumping out the same amount of air or at least making the same size boil of bubbles in the water. Any ideas on how I might trouble shoot this thing? TIA!

Do you have a relief valve at the compressor itself? If you can open that, or disconnect an airline at the compressor, and the noise remains the same, your problem is not some kind of blockage in the airlines.

That's a good idea. I don't recall there being a relief valve of any kind though. I wish I was at home today. I just didn't have the time to go inspect anything, but I could see from the house that it was still pumping air at least. I will be antsy through the day wanting to get home and check this thing out.

Open up the pump. If it's like my HB 80, the top cover unbolts, exposing the diaphragms.
When one of the diaphragms on my pump failed, the mechanism rattled around making lots of racket. Some air was still being delivered to the diffuser. I was able to get parts on Amazon, and easily install them.

This is a great reason to have a pressure gauge on the line going to the pond. Or the manifold where the lines go to the pond. You can check to see if the pressure is different. Another way to check is to plug the line with your finger while looking at the gauge. If it can't build more than a few PSI then the diaphragm is bad too. OR it could be a bearing going bad.

In central Missouri I think you can easily and safely turn off the pump and not cause any repercussions to the fish or the pond. This will allow you to trouble shoot and fix the problem especially if the pond is not frozen and does not have more than 1 " of snow cover. Since you have been running the pump daily there will be plenty of DO in the water to supply the pond with oxygen until any ice melts off. Then phytoplankton will make ample DO until the water reaches 50-55F. If your pond is not hyper-eutrophic and does not have any snow cover, you will not need to restart the aerator system until late March early April. This will give you plenty of time to get the pump 'fixed' and back into good working order. Return and let us know what you find out about your noisy pump .

What I found when I went down to the pond to open the box and inspect the pump is that it has a vibration going that worked it over and up against the edge of the box which was causing a lot of the noise. I don't know what the vibration is about but I hadn't opened the wooden container box for the past few months at least. I moved it back to the center and away from the sides and couldn't see it visibly moving but I am sure there will be creep happening again with the vibration. Now it is back to just a humming noise but one I can hear from 30 yards away on a still evening/morning that wasn't that distinct before. The boils in the pond don't seem to be troubled and appear to be getting good air flow. I am relieved to say the least knowing that the awful part of the sound was actually the rattling with the box. I will disconnect it this weekend and bring it up to the house to open up and see if there is anything visible to me that may be causing the vibration.

Good easy fix at this point. If you don't have a low pressure psi gauge (15-30psi) on the system, I suggest that you add one to the manifold before the 2 valve split. The gauge will allow you to tell if the air flow becomes less because the pressure will be measurabley more due to clogging of the diffusers. Less pressure indicates air inflow restriction - clogging of air filter. An adjustable pressure relief valve will also prevent over pressure damage to the pump the most expensive part of the system. Set the valve to 2-3 psi more that the needed pressure to operate the system. You just want to prevent over pressurization that can damage the pump

Also, if you haven't cleaned or replaced the air filter, so so asap. They need to be cleaned/washed/replaced about every quarter.

Thanks for the advice. Sounds like a good plan.

That's one thing I did do while I was checking things over after work. The filter looked pretty good but I cleaned it out the best I could anyways. I'm also going to attach some foam pieces to the floor of the box where the pump feet rest.