Vauxhall Adam Air Intake Leak

That sounds familiar! I've got a Vauxhall Adam myself and had similar symptoms last year. Check around the air filter and the flexible hose leading to the engine. Is the hissing constant, or only under load?

The hissing is definitely worse under load, almost absent at idle. I'll check those areas you mentioned. Was your issue also a leaking hose, and did you notice any engine misfire?

Yeah, mine was the intake hose; a split right where it connects to the engine block. No misfire, but the engine was definitely working harder. Honestly, if you aren't sure, I'd get a workshop to check it out, they can do a smoke test and be certain.

Thanks for the advice. I inspected the intake hose and found a split as you suggested. I've replaced it. The total repair cost me 100€. Problem solved!