Thump sound near main valve

Hello,
I'm hearing one thump near the main valve when I turn on a faucet or flush. Main valve and water heater are in the garage. It sounds like someone hitting the wall with a closed fist. It started just over a week ago. I had the city test the pressure at my home last Weds. and it was around 50 psi. Had a plumber come out later the same day (he has 30 years exp.), he also tested the pressure and it was 56, and we couldn't duplicate the problem. I have a pressure reducer valve as well as an expansion tank, I'm pretty sure they are both the original from when the house was built. Hadn't heard it for a few days, but started back up again over the weekend. It seems to be less common when we have a steady stream going (due to the freezing temps), also, it seems like if we run the water or flush within a close time period (5 minute period for example) then it doesn't happen. So that one thump at first gets it out of the way, unless the water is not running for a while, which makes me think it's pressure related. House is 10 years old, 2 stories, water heater is one month old. Any theories? I am by no means a plumber, but I would like to figure this out before calling a plumber again, unless its something i can handle on my own. Any advice would be much appreciated. Thank you in advance

Update to this: Heard the thump this morning when I flushed the main level half bath toilet, while wife was showering upstairs. Makes me wonder if the toilet could be causing it somehow, but also can be heard at times from the kitchen sink, cold or hot. Happens pretty much after the water is turned on, or toilet flushed. Kitchen sink and toilet are about 10 feet apart if that matters. Thank you

I can't back this up with concise explanations, but if I were to be onsite diagnosing your problem, I would be very suspicious of the pressure reducing valve.

What type of service entrance valve ( main water shutoff valve) do you have , gate or ball valve ?

I have a ball valve. Last night I ran the sink, toilet, shower in upstairs bath #2 and it did not make the thump. I have not heard it yet this morning: master shower and toilet were used this a.m. (located above main valve area), kitchen sink is still at a slow stream due to single digit temps, toilet on main floor has been flushed.I'm pretty confident that when I heard it yesterday morning, that the kitchen sink was at a slow stream then also.
Thank you

Could sudden changes in pressure from the street supply line cause your pressure valve to react?

I have had fill valve make loud noises and when they shut of slamming the diaphragm and making the noise. Don't know if this is your problem but open the lid if the toilet and test it to see if you can hear it louder in your tank

@DIYeverything, I'm not sure, perhaps someone else could better answer that question. The city came out and tested at my street supply, as well as the house, and both were normal pressure. The thump has started again on the main level. Last night running half bath faucet, as well as this morning in the kitchen, and running the washing machine, also on main level.
@camaroderrick73, the one thump I'm hearing is near the main valve, located in the garage, noise sounds like someone hitting the wall. However, it could still have something to do with the toilet on the main level. On occasion, I will hear 2 thumps back to back. Again, this is happening a second or 2 after I turn the water on, flush etc. If I flush, then run the sink, it will only happen when the toilet is flushed, not when I immediately run the sink. Started almost 2 weeks ago.
Thanks again

Just turned kitchen sink on, 2-3 seconds pass and I heard 2 thumps near the main valve. Flushed toilet a minute or so later and no thump. Any chance the fill valve could be the blame, even with hearing the sound when I use the kitchen sink and washing machine? I've been reading a lot of forums and some suggest starting at the fill valve. I did replace it a few months ago. Perhaps I didn't do it well enough??

Ok, heres the connections I’ve made so far. When I have the steady stream running (because of the freezing temps), the thumps are minimal if not gone. Yesterday was warmer and so no stream going, and the thump was happening, in the form of 2 thumps back to back when I turn on water, flush, etc. Also, it is not limited to my main floor, it happened when using upstairs 2nd bath last night. But, I have not heard it when the master bath is used, which is located in the area above the main.

Is this a pressure valve issue, or could it be air pockets in the pipes? Can I shut the water off, run the water out of everything, turn back on? Could that help?

Two things I would check- first check the toet fill valves and see if toilets run or if the fill valve is a quick closing style- second open the wall and see if maybe a pipe is not properly fastened( the pressure moving when you open and close things may be causing water hammer in the pipes and cause inch them to thump against Sheetrock or wood


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I am in agreement with Phishfood. I think it's the pressure reducing valve.

I was able to determine the expansion tank was bad, I bought one and attempted to replace it, but it wad tight as he!! and I didn't want to damage anything so I had a plumber come out. He agreed it was bad, and said he usually replaces the exp. tank and prv at the same time, but we'll see if it was just the tank. So far so good, but that was only about an hour ago. I ran the sink and didnt hear it. He was able to hear the thump before he got started. I will give an update in a day or so if that was indeed the issue. Thanks for all the replies!