Engine noise on new Passat

[AlanH]

Hello, first time poster here hoping for some help. I got a new Passat 6 weeks ago and have been pleased with it until the last week when I've noticed a strange engine noise. It's most prominent when the revs are at 1.4 and sounds like a low pitched buzzing noise. I took it to the local dealer who've had a look and they thought they'd mended it, but the noise is still there. Anyone experienced this with a new Passat and how did you resolve it? Cheers.