Bathroom Extractor Fan Knocking Noise on Overrun?

Hi my bathroom extractor fan is a greenwood airvac, and when i turn off the light switch the fan continues to pull air for 15mins on an overrun. However, as soon as this kicks in a loud knocking noise comes from the fan casing.

With the light on this doesnt happen, so i would assume the speed slow down causes something within the case to flap. I have cleaned out the fan but the noise still occurs.

Can anyone advise on a similar problem and/or solution?!

Thanks
Mark