I have had this problem before before with a Trappist-style Single. It turned out that the curved liquid dip tube was rotated inside the keg so that there was only a tiny gap between the bottom of the dip tube and bottom of the keg. This restricted the flow and caused the foaming. The end of the dip tube is obviously supposed to sit in the center of the keg and must have gotten off-centered as I tightened the post. When I would go to pull a pint it would make that sound that happens when a keg is about to kick and all I would get was foam.
Note that this happened with the one keg I own that is not made in Italy or US. It is an AMCYL keg made in India and has caused me a lot of problems. I would not recommend them to anyone.