Gurgling engine noise in S2 2.25 petrol

[sid]

Hi all,
I'm in the process of getting a 1960 S2 back on the road, close to a rego inspection now with only a couple of brake hoses to replace when they arrive in the post.

I've got two noises coming from the engine that I'm trying to identify, this is the first - a gurgling sound coming from the oil filler. Anyone know what it might be? Engine starts and idles well, on my limited test drives it's got what I would say is a normal amount of power.

Engine has only 18,000 original miles, so not worn out, but likely something might be blocked or perished as it's spent a lot of its life sitting in sheds as it's an ex fire truck. I changed the oil and dropped the sump last week, nothing of note there, but didn't inspect the oil pump closely. Still didn't fix the noise.

Thoughts? Video of sound below -