Lets not mess about here, your life could be at stake - tell them you won't be flying in that aircraft again. I was given the 'oh that's normal' routine when I was learning to fly on a PA28 - I refused to fly it again, tech logged it and a couple of weeks later the cylinder head crack and duff exhaust attachment (amongst other things) were finally detected. Eventually the engine had to be replaced.
You're not there to test drive their a/c for them - always use an in-date CO detector card, £2 is cheaper than a funeral!