You said one speaker. If its the same from both its normal. If its only one first reverse channels to be sure its the same signal and level you're comparing. Then if its still only one look to see what it is. Loose speaker is very unlikely- you would hear it from in front not behind. Duh. It could be anything from loose or cracked port, to filler that came loose and is flapping against the port inside.
Whatever it is, if the only way you hear it is to stick your ear back there then my advice is the same as Henny Youngman's doctor gave him when he said Hey doc it hurts when I do that: "Don't do that."
Whatever it is, if the only way you hear it is to stick your ear back there then my advice is the same as Henny Youngman's doctor gave him when he said Hey doc it hurts when I do that: "Don't do that."