Sprinkler Issue - Leak? Well Problem?

Hi Folks,
I'm not sure if I have a pvc leak somewhere, or a well problem. The symptoms are loss of pressure, heads that will pop up and down, then spray, then lose pressure and pop up and down again...
This happens in 3 of the 6 zones. The other 3 zones run great.
In 2 of the zones we can hear a loud 'knocking' or 'surging' of water / air when the pressure drops... You can even feel it "thumping" in certain parts of the yard... we tried doing some random digging where we heard the knocking and felt the 'thump thump thump' as the surge was happening - found the pvc - but after about 6 feet in length of digging 1.5 feet down, (and finding no wet spots) we figured there HAS to be a better way than just digging up the yard.
First - does this even sound like a leak? Could a leak in one zone be affecting other zones? Why would the 'thumping' be louder when it gets to zone 3 - as opposed to zones 1 and 2 (no thumping, but definitely surging / changing pressure).
We haven't found any noticible 'wet spots' in the yard to indicate a leak - even when we dug in the spots that had the most thumping, the ground was dry..
Any ideas on how to easily find a leak? And again - does this even sound like a pvc leak?
Thanks for any help.
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sounds like a malfunctioning zone valve or the solenoid within one.

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Is there any way to test the zone valves or solenoids - w/o having to replace them?
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Nothing foolproof but I would start by activating them manually. Usually there is a thumb screw or the solenoid unscrews partially then the water flows without the controller activating the valve. Look for the word Open and an arrow on the valve.
Put your hand on the valve, you should be able to feel and hear valve chatter once you are that close.
Not sure what kind of defect to expect, something that causes oscillation of the valve under certain pressure conditions. some debris caught in the valve? Rubber parts are notorious for causing noises especially diaphrams and valve seals
Any leaks after this valve would just appear like a sprinkler head to the system, it would not cause thumping.
Also try adjusting the main supply valve, make sure it is all the way open. It may also be a source of noise. Since you have the same thump on 3 branches of pipe, it may very well be this or something common to all 3

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If your water pressure is on the low side, it could be automatic drain valves, which close under sufficient pressure, opening due to dropping pressure as the tank pressure drops. Do you notice the symptoms coincide with the pump cycleing?
Bob
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