Members of the Belfast City Airport Watch campaign group gave evidence at a Stormont committee on Wednesday.
It comes as Environment Minister Mark H Durkan considers an application by the airport to remove the cap on the number of seats it can sell.
Last week a report by the Planning Appeals Committee recommended that the limitation, currently set at two million per year, should be lifted – provided that other noise control measures are put in place.
But Dr Liz Fawcett from the watchdog said some 18,000 people could be affected by noise if the expansion goes ahead.
She called for an independent regulator to be set up, which she said would strike a better balance between commercial interests and nearby residents' well-being.
“If they were permitted to do that, then you are looking at City Airport becoming one of the five noisiest airports in the UK in terms of population impact," Ms Fawcett told U105’s Frank Mitchell Phone-In on Thursday.
“It would be right up there, believe it or not, with Birmingham.
“That really does give you some context and that’s why yesterday we went to see the Department of Regional Development Committee.
“We said, whatever the outcome is in terms of what the environment minister wants to do, we don’t believe there’s good enough noise regulation.
“What we want in an independent noise regulator who will make decisions completely independent of any of the airports about all the airports, in terms of what they should be doing for noise regulation.”
In response to the Belfast City Airport Watch evidence to the committee, a spokesman for Belfast City Airport said: "There are already well established noise regulations in relation to all UK airports, of which Belfast City Airport is fully compliant.
"These were taken into account at the recent public inquiry into the removal of the airport's redundant 'seats for sale' restriction which heard evidence from all parties, including the Department for Regional Development and Belfast City Airport Watch.
"Last week's Planning Appeal Commission report recommended that the removal of the seats for sale restriction should be accompanied by additional noise controls, and the airport will be making representation to the Department of the Environment in the timeframe requested."