Obviously it is impossible for anyone to "diagnose at a distance via a fourm" but, from your description of the "whoosh" sound, there could be a joint that is beginning to show non-linear tendencies. If there is no continuous noise, when listening via a wired telephone, it is unlikely that the suspect joint would be easily found.
In the quiescent state (telephone not in use, modem/router connected) there would be no current drawn from the exchange "battery". When the telephone handset is lifted, the circuit is completed; the line is looped, current then flows around the loop. That current flow through an "iffy" joint can produce a momentary "whoosh" sound. If the current is large enough to "heal the joint" (i.e. it acts as a "wetting current") the noise will cease . . . until next time.
Does the modem/router re-train when an incoming call is made to your telephone? If yes, that is another indicator of a suspect joint.
Until such time as the fault becomes "hard", i.e. the joint has "ripened", and the noise is constant it is unlikely that Openreach would accept a fault report.
