Toyota 4Runner new A/C noise

i just had to replace the evaporator in our 2007 Toyota 4Runner that had a leak. I was told tho required taking the entire dashboard out to the firewall.
it took 2 days and cost $900 total (about $360 for the core and the rest labor and shop).

Now the A/C is cold again but I have an intermittent sound that was not present before from behind the dashboard that sounds like a hospital ventilator or Darth Vader.

The shop hooked it back up and could hear it and see a pressure fluctuation each time it occurred. They drained and refilled the coolant with no result. Now the think it is a valve/ regulator (they say there is only one in the system). so i’m wondering why this would start happening now? could the evapotrator they put in be bad? did they cause it? should they have changed that valve at the same time? and if it will cost me $900 again?!

Sounds really suspicious that you had a bad evaporator on an 07 Toyota. They are not a normal failure item. What were the symptoms before they got a hold of it?

An expansion valve flutter is a possibility. As to why it is happening I cannot tell you as there are so many unknowns.
If changing an evaporator I would have probably done the expansion valve at the same time.

An '07 Toyota seems a bit young to suffer a leaky evaporator so that does come across as unusual.