This is a bit of a weird one. At motorway speeds I get a mild resonance through the car, comes and goes in about 3 second intervals. 3 seconds resonance, 3 seconds smooth. Felt more through the seat than the steering wheel. Feels like a mildly out of balance rear wheel, except for the fact that fades in and out and is not continuous.

 

Any ideas?

It could be a bunch of reasons but it sounds very similar to the "Saw toothing" problem I & many others had on S2 Superbs which resulted in an on/off resonance at 60-70mph caused by cheaper tyre makes &/or wheel geometry problems. 

Check the tyre wear, tyre pressure (front & back) & have the front & rear wheel tracking / camber checked. Out of whack tracking / camber can result in tyre rumbling which is more audible on certain types of road types e.g. concrete sections of motorway.  A 4 wheel alignment & tyre change finally sorted my 2010 S2.

 

Not sure if this problem has been carried over to current S3 models but it is entirely possible for a similar problem to exist - especially if cheaper tyres are fitted. My current S3 rear wheel camber was out of whack which resulted in dramatic inside rear tyre wear down to the canvas & a slow puncture while the outside tyre tread depth was still at 3-4mm so don't take anything for granted. A 4-wheel alignment has hopefully cured the problem.

It could also be a rumbling wheel bearing - mileage? Jack the car up & spin each wheel & see if you can detect any noises or grumbling. However, wheel bearing noise is normally a more continuous rumbling & not an on / off resonance.      

Cheers Colin, tyre wear is even and rear tyres are a mid range so not junk.  There is not much life left in them so may change them first and see if it helps. But good suggestions regarding alignment.