I had this problem 2 years ago. What was started as a cricket noise turned into 200euro fixed with labor included at
Bavarian Technik. All belts were replaced at first as mechanic thought it was the belts and CSP(crankshaft pulley) were replaced 10k ago.
Turned out the CSB replacement were
not a good quality.
BMW wanted 370 euros for this part.
After the replacement, nothing can be beat the sound of an enthusiastic engine. Hope that helps!