What could it be, when I hear this noise like a water faucet being turned on & off w/in seconds, just within my bathroom wall, which is @ the firewall adjacent to the condo. Next door - while no water in my condo is running anywhere? It almost sounds like a quick spraying of water like you would hear from a hose, but it stops within 10 - 15 seconds. I just heard it (10 AM), but have heard it a couple times @ night about 10 PM, also when no water was running anywhere in our condo. We can hear no noise from our next door neighbors ever - supposedly the condos are separated by cinderblock firewalls. We have lived here over 15years and never noticed this before.

Hissing would indicate a pressure leak inside the wall. To check for a hidden leak go out to the water meter, (you will have to check both your meter and your neighbors) look for the little hand below the read out. With all your fixtures shut off you should not see the little hand moving.
If it is creeping , even a little bit, you have a hidden pressure leak.
Call building maintenance or a plumber to check it out.
Good luck, Tom

It could be your neighbors toilet tank filling up, if the flushvalve is seeping water it will start filling when the water level gets to low, have your neighbor check their toilets. Goodluck.

Also, it could be regenerating R.O. system and / or refilling icemaker...

Also, it could be regenerating R.O. system and / or refilling icemaker...

What is a regen. RO system?

What is a regen. RO system?

regen= regeneration
RO system= Reverse Osmosis Systen
Did you check the water meters as I suggested?
Back to you, Tom

R.O. is an abbreviation for Reverse Osmosis water filtration system that is usually installed under your kitchen sink.