try different speeds, we had a really nasty dip near work (they finally fixed it 2 weeks ago, its been there for years) and if I went thru it at anything over 50 km/hr my hard plastic mudflaps on the front wheels would touch down when the struts would compress. It annoyed me so much that I changed the mudflaps to a softer rubber one. Can you tell roughly what part of the car the noise is coming from?
Somewhere there's a DC10 struggling to get off the ground. I'm not a fan of wings on street cars, I like low key spoilers, but not something that you have to custom order from Boeing.