Mk111, LAB was on the verge of saying the same thing I bet he ered on the cautious side.
If you do have an older boyer, it can advance if it doesn't have full voltage making the engine kick back and perhaps causing the noise.
Is the fuel/air mix being blown back out of the carb? or squirting out somewhere else?
AntrimMan noted the noise could be the Anti Backfire device doing its job and making a noise.
A good battery, good cables and connections are a must with Mk3s. Mine has a standard starter motor and starts fine when the engine is warm but some times I have to kick it when cold.
Get the best battery with the most CCA you can afford and it will last a long time.
(all the above learnt on this site, Thanks)
Regards
graeme