1.8 20v engine sounding like a diesel?

[Spencer11]

I returned from a long journey in my passat lastnight, the car drove 300 miles faultless i must say.

My passat is a 1999 model 1.8 20v se.

This morning i started the car to go to work and it sounds like a diesel but its a petrol! Its never done this before.
I called a friend who knows a little about cars to come have a look and even he asked me if i was sure my passat wasnt a diesel.
Anyway he checked oil level and had a quick look around the engine and couldnt see anything out of the ordinary.
So he started the car and let it warm up a little and then took it on a short drive and when he returned it didnt sound like a diesel anymore, it just sounded normal and quiet running.

Then as i was talking to him and the car was idle just ticking over it started sounding like a diesel again!!

I havent taken the car with me to work as i didnt want to damage the engine.

Anyone have any ideas what could be the cause of this?