Something is blocking the free flow of water down one or both waste pipes. First, check the outside drain. Is it blocked? Fill the wash basin with water, then watch what happens in the outside drain when you pull the plug out (or get a willing assistant to watch for you). If the water level in the drain rises noticeably (say, to the top of the drain), then slowly subsides, the blockage is probably there. The drain needs cleaning out. The gurgling noise is caused by the end of the waste pipe being under water whilst it is emptying. Second, if that doesn't seem to be the problem, it's probably the inside of one or both waste pipes being lined (and almost blocked) with soap and all the other detritus that goes down waste pipes. The flow of water - especially in the shower pipe - is too slow to wash away all the soap, etc., which allows it all to build up inside and slow things down. How to fix it? The sure and certain way is to take the pipes apart (if they're modern plastic, they're probably a screw fit or a push fit) and give them a good clean out. The easy, but not so thorough, way is buy a bottle of drain cleaner and use that. Might take more than one application. Best of luck.