Check engine light came on. Code indicated a "slight" vacuum leak. Searching internet recommended changing the gas cap as the first step.

Purchased new cap from dealer and reset MIL. The light came back on a few days later. In addition, while driving down the highway, there was periodic "pop" from under the hood or under the dash. It sounded like a relief valve or something. However, we also noticed that everytime the noise happened, the AC would blow hot air for a few seconds, then it would cool off again. This "pop" was not continuous, but periodic, about every 15-20 minutes.

Any suggestions on "where" to start the search for vauum leak? What is the best method for locating such a leak?

Thanks in advance for your suggestions.