We had antifreeze added to our heating pipes several years ago. Recently we changed oil companies and they "added" more antifreeze. Now we have a terrible tapping sound in the pipes when they are hot --- the noise does not happen when we turn the system on. Only when everything is heated up. Could the problem be caused by antifreeze? If the new oil company used a different antifreese could this contribute to a problem?

When it comes to antifreeze there are many different types and I hope you have the correct one for your boiler system. The wrong type can act as a acid and really cause problems. The tap sound sounds like a expansion problem and you might want to get that checked into by your service company. There is not much a homeowner can do with a sealed antifreeze system.