how good are the chemicals you can buy?

Bought an old house with existing gch system. Had various additions/alterations/updates to our system over the last 9 yrs. All done by a "professional". However, we have had a couple of minor leaks from the old pipes. Now need to flush the system and am doing a manual flush as engineer says power flush may show up further possible leaks. Have seen a chemical that you can put in system to stop small leaks. Does this work? Want to add with the inhibitor. Don't say replace pipes!! Also there is a noise reducing additive. Does that work? Sounds too good to be true - is it??