to check for back pressure probably.

The revs dropping when the oil cap removed is normally a sign of a good vacuum.

If it growls, or is smoking when cap removed It is most likely a breather problem. Use it as a bargaining chip and get some £££ off.

[Edited on 11-02-2016 by johnhara1]

Why did he take oil cap off while it was running :/

Cars are very noisy if you take the cap off whilst it's running. Don't put your face over the top! ;)

When the cap was on they're was nothing it was normal, as soon as he took the cap off there this fairly loud knocking noise coming from the engine, but when he put it back on it stopped? I'm test driving it today should I not bother with it or is it not a problem at all?

all cars will make a noise when the oil cap is removed.
lots of moving parts under there and oil being lashed around.

could you hear anything when the oil cap was on?

Head gasket went on my previous car so now buying a new one, went and viewed it today Seat Ibiza 1.4 turbo and me and my father in law and we were looking around it and when the engine was on the seller took off the oil cap abs it started making this awful noise knocking sound started but when he put the cap on it stopped, is this a massive red light? We are driving it tomorrow for a test drive so I need to know what I should do. Let me know when you can, cheers.