Hello Paul,
One of the first ones i saw, the customer decided to leave it, then one day (a few months later), I got a call to say he had a funny noise & the engine stopped. One of the flaps had been ingested by the engine, breaking two valves & sticking them sideways into the top of the piston. The second replacement engine that he had also developed a rattle after a while, which he hadn't noticed until I pointed it out to him after a service. On that one I removed the flaps all together & blamked off the holes & its' been driving fine since. On another one I replaced the manifold with a new one, which I think cost around £500-£600, but the driver thought it better to replace it than have the worry of it going bang at any moment.
Regards, Andrew.