Re: Asking for advise
Alfasud.ti is correct.
In general unleaded petrol has a lower octane level than the leaded stuff, so the strange noises you are hearing is the pinking (spelling???) that is caused by the engine.
You possibly have to retard the timing about 1/2 or 1 degree and the sound will go away.
There is an easy way to check. If the car is idling you should not notice the noise, only when the engine is under strain, for instance when you are driving uphill, should you hear the noise.
In the long term it is very damaging to the motor, so get it done as soon as possible, otherwise you could end up with burnt valves.
Boxer motors have cast iron blocks, but the heads are aluminium, so the valve seals etc. will handle the unleaded fuel fine in all other respects