Knocker

I picked up a 2002 1.8 TF with 52k miles last weekend. My dad has had a TF for a few years now and i've been trying to buy it from him as he barely uses it but he was having none of it so I had to buy my own. I was told it had a bit of an exhaust blow by the previous owner before I bought it so its something I was aware of, the check engine light is also on. I drove the car 150 miles back home without any issues at all other than the slightly loud exhaust when accelerating (it sounds fine when the car is cruising).

I jacked the car up next day so I could have a look at the flexi section of the exhaust and when the the engine was started the blowing exhasut sound had gone, i'd had a look at all the sections of the exhaust system I could get to under the car and can't find any sections that are leaking. I've put the car back down on its wheels and started it up again and the blowing noise is still gone. As it was getting on in the day and I was losing light I had to give up on it for the day so haven't been able to look any further into it.

Does anyone have any suggestions as to what this could be? I'm going to take the car out for a short drive at the weekend to see if the noise comes back. Could worn or broken gearbox / engine mounts cause this if the engine is able to flex slightly and put pressure on the exhaust ssytem joints possibly opening them up slightly?

I'm also not sure if the head gasket has been done. The car runs fine, it didn't lose any coolant when I drove it home and there no mayo in the coolant or on the oil filler cap. Is it worth replacing the head gasket as a precaution or just leave it until theres an isue and it actually needs replacing?