Squeaking noise from pipe - no internal stopcock?

Last night while doing the washing up we both heard a high-pitched squeaking sound, not very loud, and almost like air being let out of a balloon or a pierced latex glove. It lasted about a second and then petered out, and sounded like air escaping under pressure. We didn't hear it again until about five minutes later, again only lasting for a couple of seconds, and haven't heard it at all since. The house is a rented maisonette that used to be a workshop, and was converted about five years ago. Next door is the same but mirrored, with the downstairs bathrooms back-to-back, and has its own water meter. There is a gas boiler that runs only the central heating and hot water (at the moment turned to just hot water) which lights perfectly well and is at the right pressure.
I phoned the letting agency up in the morning to ask what it might be and to try and find the location of the interior stopcock, as we're going away at the weekend and I didn't want to come back to an indoor swimming pool if it does sound like a leak or problem with the pipes. They haven't got back to me yet. I know where the main shut-offs for the electricity and gas are, and as the water is on a meter I know the external stopcock is in the same pit, but I can't find an internal one anywhere. In addition to that I'm not sure that I can turn the external one off - it's a Welsh Water one, and they've said that I have to book someone to come out in five working days, or pay to have them come out sooner (apparently I can't do it myself as it's their property). Under the sink is just the isolator for the washing machine, and a hole in another cupboard just had an odd bit of insulation lagging in it. The pipes in every room disappear into the walls with no visible isolators for themselves (a case of making things look pretty more than easily accessible, it seems) and the bottom of the baths (one upstairs and one downstairs) are panelled up with wood and sealed. I know the stopcock should be on the ground floor somewhere, but there's no obvious point at which you can see the mains water enter the building.
Do houses have both an internal and external stopcock, or is it likely that there simply isn't one?
Any ideas on what the odd squeaking sound was would be greatly appreciated too!

 

Found the external stop tap earlier, so at least there's one option if it starts to rain indoors! Had a dig around under the kitchen sink again and all I can see is the isolator for the washing machine. Everything else disappears into the back of the cupboard and into a stud wall, very neat but pretty hopeless for me! Another promising-looking panel just had the top of the waste water pipe poking out of a solid concrete floor, supplied by the same sink waste pipe I saw re-emerging from the stud wall. Seems a bit of a faff weaving waste pipe in and out of walls when the waste channel is only half a cupboard away.

I have a horrible feeling I'm not going to know where it is before I go away at the weekend. Is it worth turning the water off at the mains while I'm away this weekend (Friday, Saturday, Sunday)? Sod's Law states I probably should, but I haven't heard the mystery squeak again so far. Is it just an odd but normal pipe noise, you reckon?

 

Brilliant! I'll try and have another dig around the meter pit and see if I can find that tool you mentioned. Thanks for you help!