Ive heard about sightings of a sauropod in th Congo. It is rumored to be Mokele-Mbembe. Is it possible that a dinosaur could still be alive today and if so could it go unnoticed for so long?

Sorry: the answer is simply no.  As a very passionate sauropod lover myself, I would love this to be true, but there really isn't a shred of meaningful evidence.  And if any sauropod lineage has survived from the Cretaceous until now, we'd expect to have found Tertairy-era fossils of it.

This is not a new rumour - see
http://en.wikipedia.org/wiki/Mokele-mbembe

Big things can certainly be hard to spot in dense enviornments, but remember than even the smallest sauropods were generally twice the size of a modern elephnat. That's not an easy animal to hide or miss, especially as people have been looking for a long time.