how can i fix my 2oo4 1.6 renault megan vvt dynamique?

i have an intermittant whering noise when starting up the car. the garage cant seem to find the fault as it does not happen all the time. it is loud and sounds similar to a childs bike when they have those things attached to the forks that rub against the spokes to make a loud clicking/flapping noise . this noise stops after a few seconds of the engine starting