Could my cassette be making a grinding noise? Does it need to be replaced?

2021 Jamis Coda S2. Have ridden it in the past 3 months approximately 700 miles. Within the last 50 miles, I started hearing a grinding noise when riding. I don’t know much about bikes, so I cleaned up the chain etc. but the grinding continued to get slightly worse. After research, I figured it was something to do with the bottom bracket or bearings. I decided to bring the bike in to my LBS because that stuff is over my head. LBS owner and mechanic looked at the bike, owner the. took it for a ride, and said “the bike is fine, the noise is from the cassette, but I would ride the shit out of it before changing it”. He told me I didn’t need a tune up or anything, he told me to come back when the chain starts to slip – that I’ll know when I really need to replace the cassette…. Is it true that the cassette could be making that noise or is it possible the LBS just did not want my business/to work on my bike? I’m very happy they didn’t take my money and do a fake “tune up” or other repair, but I also don’t want to damage my bike further if it actually does need a real repair…. Advice please?