I have read in an old post on this forum about noise problems with the "KMC X1 Missing Link". It was suggested that it was due to wear, because it is not supposed to be used once it has been opened. Although I read the instructions of manufactures and listen the opinions of experts and non experts and use common sense as my final criterion.
In defense of the Missing Link I have never had any problem with it for 17000 km on a KMC X1 chain, chain that I replaced when it was close to 1% wear. I have opened and closed the same Missing Link many times to clean the chain in boiling soapy water. Every time that I opened or closed it there was a very tight fitting and click noise, so I considered it was safe to re-use it. When I replaced the chain I inspected the Missing Link and its wear was similar to the other links of the chain. I put a new Missing Link with the new chain and back in business.