Qubo MJ engine, 79000, frequently changed oil, developed an increasingly loud tick / click / pop which synced with engine revs. Loud at idle, sometimes quiter when cold. I dropped off the auxiliary belt, still there. Took to a local garage who removed and re seated an injector that had been blowing at the seal. All good now, modest cost and it was probably doing it a little on and off for a while. Nice and smooth and quiet engine again [emoji4]

I had exactly the same last year , the popping is the escaped gases forming into a black like tar which goes rock hard on cooling , this then comes under pressure when started again and forms the explosion
I was quoted I recall £350 for the 4 to be done , I decided to do it myself , a bit of a scary job as you cannot afford to let any foreign bodies fall into the cylinders , mine where really gunked and i got loads of the black stuff falling into the pots along with brass and WD40 , you definately have to have the tool for the job , also my wet and dry vac with breather hose attached was a godsend to hoover out the cylinders !
It cost me £8.80 for the seals and a crate of fosters for the lad who lent me the tool , happy days