Yesterday morning for the first time I heard a loud noise during maybe 10s before the rpm dropped from 1500 to normal idle. After that the engine felt fine and I experienced no problems at all. (Apart from that pothole later on which was a bit harder than expected.)
So what could this be? Some pump?
Loud noise? Was it short explosion/shot like or loud rattling during whole heat-up time? In former case, can you replicate via eg. quick stomping on accel pedal? If your car is older one, i'd check injector seals. If later, yes, possible rubbing of cold exhaust parts. And if "whistling", belts.
It was a scraping or rattling sound which lasted about 10 seconds. It ended when the rpm dropped and did not happen again. Certainly nothing like a pop or a bang. It could very well be related to the heat shield on exhaust manifold. Maybe if I hear it again I'll get more clues (like detecting a resonance frequency).