Perhaps. Welcome to rental gear. I've seen this before and yes the slow hissing did stop as I breathed the air down in the tank. Here is they way I see it - renting means paying. Payment is given to rent a properly functioning piece of gear. Demand another one or dive with the hiss.
The intermediate pressure (IP) is a bit high, some regs creep up over time, or the second stage cracking effort needs adjusting. Most likely, the reg may need servicing. In my traveling experience rental regs are not serviced often enough. Many dive operators do not have a service technician which means regs simply don't get serviced when they should be.
Or it could be that the reg was just serviced and the 2nd stage poppet has not bedded in yet. Not all shops cycle there regs after servicing to ensure the poppet seat beds in, and I imagine that a shop or operator with number of rental regs would be serviced and pushed back out for use without a fine detail like this being done....on the bench if the IP locks up normally the reg is put back in service. The most likely time for a reg issue/failure is fairly soon after service.
For the OP...if you experience this in the future, try taking a few breaths off the 2nd stage that is hissing, try pushing the purge button a few times briefly in succession, and you can also try shaking or tapping the 2nd stage against the palm of your hand (not too hard though), and/or submerging it....if the hissing does not stop and you are in the vicinity of where you rented the reg set from you should ask for a different reg and mount and test the new one too.
I do not recommend pulling the reg off the hose and meddling with the adjustment unless one owns the equipment or is asked to do it for someone who owns the equipment. By doing this you are accepting full liability/accountability for damage of the equipment, and any injury or death that may result. If it is felt the equipment is not within spec/functioning to expectations take it back and discuss with the rental shop.
Often, though, this issue will manifest when one is not in a position to exchange for a different reg set. In this case, if the 2nd stage is just hissing, while an annoyance, it should not materially impact a dive...you may have slightly reduced time underwater due to the air escaping but the regulator should breath fine. One way to handle this if it is the octo and the primary 2nd stage is ok, is that you can switch to the octo and breath off of that as the primary, that way the slowly escaping air goes into your mouth/lungs instead of the water column.
While this type of malfunction is a problem, from a go/no go perspective this is not a "no go" problem unless the hissing progressively gets worse or becomes a full-on free flow that will not stop.
Of course, if you ever have concern about whether a piece of equipment is functioning properly you can abort the dive/plan. There is no recreational dive that is so important that one should take unnecessary risks and proceed with a dive while questioning if your gear is working properly or not....you either believe and trust that your gear is working properly and proceed with a dive or you accept that it is not and postpone the dive until you can obtain gear that you trust to be properly functioning or abort the plan completely.
-Z