Loud uneven engine noise when starting
 

Hi, I bought this 2007 volvo s40 recently and I noticed there is a loud, uneven sound when staring, ONLY after it's not been driven for a couple hours. For example, Every morning when I start the car, it has the loud, uneven, and motorcycle-ish (but not that loud) sound. It will last for 10-15 seconds and then it will settle down, the loud noise will go away. If I start the car again in 20min, no loud noise. So just the starting that my car has not driven for hours has the noise.

One time the noise was particularly loud and shortly after I started it and drove for like 10 seconds, the engine stopped working. I couldnt accelerate and the car was jerking a little. I could drive only 5 miles per hour. But everything worked fine after the car was restarted. However, I was using regular gas for this time.

Check engine light is not on, oil is fine, belts are relatively new, using premium gas now, new spark plugs, 144,xxx miles. The mechanic said the engine stopped working was because of the regular gas, but he did not know why my car has the loud engine noise. Does anyone have similar problems before and can tell me should I worry about it? Pretty thanks!!

Could it be your water pump bearing?
Try a cold start with you listening from the engine bay and hood up.
You'l need someone else to start the car at your direction.
Another thought was maybe your oil pump is struggling to get enough oil pressure momentarily at cold start-up. Is the oil light on whilst the noise is occurring?

Low octane fuel will not cause an issue like that.

When you say belts are new, does that include timing belt? If so, were the rollers and water pump done at the same time? Do you have proff that the rollers were changed?