You are talking about condensation trails(contrails are frozen exhaust and ambient humidity), they form when the low pressure causes the surrounding humid air to condense. They are usually seen on humid days and can form anywhere on the aircraft, but tend to be most prominent over the areas of highest lift/ lowest pressure.
The can also form in the intakes as the fan blades act much like small wings.
http://www.airliners.net/open.file?i...1279594&size=L