MX 5 NB FL exhaust problems

Hello, I have a MX5 NB FL 2003 with 25000Km. The car is about 5 months in my possession and I noticed some time ago that the car makes a kind of re-ignition in the exhaust after driving. So about 10-30 sec after switching off comes another audible “plopp” out of the exhaust. Since I have not got the car so long, I do not know if this was always there or if that is normal. Then I drove into the workshop and asked once if they could measure the ignition coil, we 1. Is the sound normal or do I have to be afraid of my cat? 2. Bzw prefer to let the ignition coil be measured? Still for information, I have had a flapper grouse from the vicinity of the cat some time ago when I was standing at the current engine (at the traffic light). But it is gone again and it seems to have been only a heat protection sheet that has sunk in the stand. cht exclude that it was the cat who has already said goodbye Stand speed is quiet and gas is also accepted well from 4000 rpm. Also from 100km/h he does not make sluggish….can also be subjective that I don’t notice the difference…don’t have the car as said so long. So everything still speaks for an intact cat or ignition coil. mfg