1999 Forester - strange sound coming from engine

I am considering buying a 1999 Subaru Forester L. It seems to run fine but if you pull out the oil dipstick while it's running there is a sound coming from inside the engine through the dipstick tube. It's almost like something is bouncing around inside the engine. If you put the dipstick back in, the sound goes away. I asked the seller about it and he immediately told me that it's normal and all Subaru 2.5 liter engines make that sound. He said that it's just the oil gurgling and bubbling inside the engine and your not going to hear it when the dipstick is in. Does anyone know if this sound is really normal or is the guy full of it?

The vehicle is a 1999 Subaru Forester L with a 5 speed manual transmission, the 2.5 liter engine and only 50,000 miles.