Hello and welcome to JustAnswer!
If the noise is always coming from the same area then it's more likely to be something in place that is shifting or making a noise because of body flex and stress, rather than something moving around in the headliner.
If the noise is above the headliner then the best thing to do would be remove the headliner and start with a visual inspect. If nothing can be seen then you can have a helper drive the car while someone in the back seat listens for the noise. It could be coming from something like a loose sunroof assembly, or it could be coming from structural bracing. There are structural braces that the roof sheet metal attaches to, you may have something flexing and causing the noise because of a broken spot weld, etc. With the noise being above the headliner and happening during times of body flex it sounds like the problem is being caused by something structural.
Thanks for your reply, can you provide me with a diagram and procedure for removing a headliner in an SRX. Are there any special tools needed for this job?
I am getting ready to try this out. I will get back with you shortly.