Have you checked your belt for signs of wear? Is that the original belt?...I would hope not, but you never know. It's probably time to replace it. If the belt happens to be in good shape, then check to make sure you even have power steering fluid in your reservoir.
One more thing, "seems" doesn't make me feel good about your method of checking tension. lol Check it for proper deflection. Also check the pulley for any signs of the rubber melting to it. Man, there's a lot of things to check for and eyeballing it is only the first minute's thing to do.
Does this happen when the car is cold or after its warmed up? It's obviously cold out now and those belts will harden until they soften up from the friction. So, tell us more