Peugeot 307 diesel boost issue

Those symptoms do sound like a boost issue. I had something similar happen in my old car. Have you checked the vacuum lines to the turbo actuator? A leak there could cause the hissing and power loss. Is it throwing any specific error codes? Knowing the code could really help narrow down if it's the actuator itself or something else in the boost control system. Could be a faulty pressure sensor too.

I haven't checked the vacuum lines myself, as I'm not entirely confident in my ability to do so without causing further issues. I retrieved one error code related to the boost pressure. If the issue is indeed the actuator valve, is it possible to repair it, or is replacement the only viable solution? Are there specific tests I can perform (perhaps with a multimeter) to confirm the electrical bypass valve is defective before committing to a potentially unnecessary replacement?

If you're not comfortable messing with the vacuum lines, I'd advise against it. Best not to make things worse! If the error code points to boost pressure, and you're hearing those noises, it really does sound like the turbo actuator. Honestly, repairing those actuators can be tricky and might not last. Usually, replacement is the best way to go for a long-term fix. As for testing the electrical bypass valve, that's a bit beyond my knowledge. Best bet would be to take it to a workshop. They'll have the proper diagnostic tools to pinpoint the exact issue.

I appreciate the advice. You've confirmed my suspicions. I've taken it to a reputable workshop, and they've diagnosed a defective electrical bypass valve, just as I suspected. The repair, including the new actuator, will cost approximately 250€. Thank you for your input. It's good to know I was on the right track!