Sweet looking machine, one I haven't seen before
Anyway, a machine with a hopper payout will attempt to payout for about 5 seconds - it only knows when a coin has been paid out as it passes through an optical sensor [opto]. If the opto doesn't sense a coin passing thorugh within approx 5 seconds, it does a 'CALL MANAGER', or 'IOU £XX' message. I am not sure about tube payout though. I'd guess, if say £100 were won, it would 'attempt' to pay out £100, through pulsing £100 times, and if it paid out less, you should have had someone to witness payout.
I don't think it would continue to attempt paying out as such...
Also, if as I would expect, the 'payout' had been disconnected, I fail to see why it would make a 'paying out' noise [like a banging noise], as I'd assume the tubes would have been disconnected?
If it is making an obvious banging noise, like it is trying to pay out - and there are coins in the tubes - but none are coming out, you may need replacement solenoids for the tubes.
If it was sold to you as fully working, but isn't paying out, I'd ask for a repair from whoever it was purchased from. If it was sold as 'payout disconnected', then you could always ask them how to reconnect it
As said above, I don't have any experience of MPS2 and wouldn't know how to reconnect. I'd assume there would be a connector on the MPS2 mainboard called something like 'Coin Handling', 'Coin Mech', 'Coin Payout' - something like that - and that the plug from the mech wouldn't be connected to it :S