Hi, a new member here with a question!

I have an old S40, 1.8 – 98 which has done just less than 50k miles. I recently had it fully serviced and as I drove home from the garage the engine started to produce a strange sound. I took it back to the garage but obviously they can’t reproduce the problem…

When the engine is cold everything is fine and it runs like a dream but after 5-10min when the engine goes above 80c the noise starts. The noise is there if the engine is running at 2100RPM or if I put some load on the car, for example if I try to accelerate fast from 20mph in 3rd gear. Apart from those two scenarios it runs really smooth. I can reproduce this 100% of the time.

The noise it self is fairly loud (if I turn up the music I can till hear it). It sounds a little bit like small explosions or like a hot rod car.

My first thought was that it is misfiring, maybe a vacuum leak or something but the check engine light doesn’t come on which I think it would if it was misfiring?

Any ideas about what it might be?

Thank you