Yesterday the only problem I had with my w123 was the fact that it was losing vacuum when parked, so after checking all the doors and reservoir I decided to check the pipe going to the intake manifold. I removed the complete air filter housing and then the rubber pipes underneath, cleaned them all up, check the joint and put it all back together.

Now the car is backfiring or popping. Its a strange issue as it doesn't happen until the car has done about 30miles, its not really evident when in neutral and revving the engine and it seems to come and go it even seems like its works going uphill and not so bad downhill. Its more evident at low rpm and the oil pressure now seems to be more active then before. It is not effecting the performance.

Any ideas what I have done or what it could be?