Hey guys. So last weekend I got my car booked in to get it fully serviced (changing timing belt, accessory belts, water pump, sparkies, you know the rest..). And jst the other day I installed a new exhaust allowing me to hear my revs more clearly. I've noticed that when the car is idling the car would increase and decrease its revs. To make it more clear, my normal idle is usually around 700rpm. This would then fall to like 400-500 or something around there and then rise to just under 1000 but then ease back to the normal 700. Now idk if this is normal or not but its worrying as ive never noticed it before.

I would think that the mechanic disconnected the battery for a while and caused the ecu to be reset? Which means the car is relearning everything? Im not totally sure.

What do you guys think?