2012 California 30 - Strange engine noise

Hi, I have a strange purring/gurgling sound coming from the engine between 1500-3000rpm.
It's louder when just a small amount of throttle is used but seems to be drowned out as the revs climb.

I did put UK 97 Octane fuel in as 99 was not available, this may just be coincidence as the noise doesn't sound like the pinking sound that I've heard in other cars.
Any help gratefully received.

 

Hi Andrew with noises it's quite hard to tell unless I hear them myself but I just wanted to ask you if your 100% sure it's from the engine and not the exhaust system at all because the exhaust, mainly blackbox and valves, making some noise is a fairly common thing and then they could require replacing