They are either on the bottom or on the surface. There is really no way they could trim the vessel for neutral buoyancy.
As far as the noises are concerned, bear in mind that they are in the vicinity of 52,000 tons of rusting and slowly-collapsing Titanic wreckage which is swept with pretty strong currents. It creaks and rattles and bangs regularly. If the bangs were rhythmic they'd be easier to identify as coming from the Titan, but I'd think there would be more excitement if that were the case. Similarly, they are in the vicinity of the Titanic debris field, so active sonar won't help if they are on the bottom in that mess. Even imaging sonar may not be able to resolve anything in that field due to the debris. They are going to have to visually go over the entire area of the wreck to see if the Titan is anywhere in it.
If they were intact and stuck on the bottom, fouled on wreckage or something, I would think that they'd be tapping away on the hull trying to get noticed. For that matter, that would also help if they were at the surface too, but they may not have realized that. There is no evidence that they imploded as that would be an obvious and loud event. In my mind, they either had a major leak and sank, or they lost power and de-ballasted, and are floating around on the surface somewhere.
If they made it to the surface, they may not have thought to make noise by tapping, figuring that they were at the surface not far from the support vessel and that they would be seen eventually. Maybe they popped up within line of sight of the support vessel, but unless they had very attentive and lucky deck watches looking for a mostly submerged small white sub, it's very possible they just drifted away with the current. It seems that no one was particularly worried by the loss of contact until after their scheduled return time, which means they could have traveled with the currents for nearly 8 hours before anyone thought to look for them. They could have been many miles away by then. At that point, they only had a few hours of daylight for a visual search, and airborne searches probably didn't start in earnest until the next morning. They could have drifted quite far by that point.
They either will be found on the bottom eventually, or they may wash up on some distant shore in months or years, or they simply may never be found. In any instance I'm sorry to say that I think finding them alive is no longer possible.
Also, in case they are on the ocean floor, it is freezing cold. Drinking water is probably also delpeted I think. And what about a toilet? I don't really want to imagine the circumstances... just horrible
