I don't see others jumping on board here, so yes, likely as anything the water pump. If it wasn't leaking coolant or the bushing screaming then maybe the impeller is slipping on the shaft.

The general consensus in the old Volvo community is that the Hepu pump is the most reliable replacement for the money. The Hepu has a cast impeller, while the GMB is a stamped impeller. In theory, a bladed stamped impellor could be more flow efficient, but the pressed attachment may be less reliable to stay attached on the shaft. Way back, reportedly the GMB impellers could occasionally separate off the shaft, but I've not heard of that in recent years. I note the most recent version of the Volvo pump also appears to now have a stamped impeller with blades, aleit it with a more solid looking center.

In the past I've used a number of both GMB and Hepu pumps with okay results and given a choice would choose Hepu. Rockauto doesn't carry Hepu at the moment, but does have the GMB at a very affordable price, so FCPEuro with their lifetime guarantee for the Hepu would be my preferred choice, although Eeuroparts has the Hepu for slightly less at the moment. The Genuine Volvo is simply not worth double the money.

There have been recent comments here about the rough casting quality of the Hepu pumps compared to the Hepus way back. It's hard to imagine a leak developing with a properly clamped hose and a proper fibre gasket (not just a thin paper gasket). No gasket sealer is called for by Volvo other than a wipe of petroleum jelly on the top of the upper mushroom rubber gasket so it doesn't fold over in final positioning. I use a hylomar-type, completely non-drying gasket sealer on the block gasket for a guaranteed seal and more importantly so the pump can be removed and re-positioned as needed later without needing a new gasket. Doing this, I've been able to remove and reseal the upper gasket when needed without having to replace the main gasket. The main thing is to force the pump up high under the head flange for final tightening and as an added bonus to put a wipe of RTV sealant into the crack between the head flange and the pump to keep valve cover oil leaks and spills from getting onto and softening the upper mushroom gasket.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now