Renault VelSatis Power Loss Whistling Noise

Sounds familiar. I had a similar issue with my old Vauxhall Insignia, a diesel too. Same kind of symptoms: check engine light, power loss, weird noises. In my case, it turned out to be related to the turbocharger system. Have you checked the turbo wastegate or maybe the pressure sensor? Those are often linked to boost control problems. It could also be a faulty turbocharger. What error codes are you getting? That would give more information to start with.

Thanks for the quick reply! I haven't actually checked the error codes yet, to be honest. I don't have one of those code reader things. But you mentioned the turbo wastegate and pressure sensor... are those easy to check myself, or would I need special tools and knowledge? Also, how did you eventually figure out what was wrong with your Insignia? Was it a pain to fix?

Okay, without the error codes, it's a bit like guessing in the dark. You can try visually inspecting the turbo wastegate linkage for any obvious damage or disconnection, but honestly, without proper tools and experience, it's tricky to diagnose properly. I'd strongly suggest getting the codes read. With my Insignia, I tried a few things myself, but eventually, I had to bite the bullet and take it to a workshop. They diagnosed a faulty pressure sensor and some damaged vacuum lines which were causing a leak in the turbo system. Based on your description, it does sound similar, especially the whistling noise. I think taking it to the workshop to be sure is the safest bet.

Thanks so much for the advice! I think you're right, I'm probably better off taking it to a workshop to get it properly diagnosed. I just wanted to get some ideas before I went in, so I didn't sound completely clueless! Anyway, it was indeed the electrical bypass valve like I suspected and the garage has charged 250€ incl. parts and labour.