Citroen C5 front end rumble

I bought the car just two months ago. When I test drove the car which had 25,000 miles on the clock, it had a strange rumbling noise from the front end which the salesman said was due to it having been standing on the forecourt for a while and not driven. After taking delivery of the car the noise continued and after being test driven by a mechanic it was decided that it was due to tyre road noise as the front tyres had been replaced by those not recommended. The tyres were changed but the problem remained. The car has just been in for two days as the garage agrees there is a rumble, however, no problems can be found. They test drove another C5 and state that it also has the same rumble to try and prove that it is normal. Has anyone else had this problem. It spoils the driving pleasure as I can always hear it, and it becomes louder when under braking when the load moves forward. I am convinced it is a suspension problem but the dealer says there is nothing wrong with the car.