Try a quick spritz of carb cleaner. If there's a leak, the carb cleaner will induce higher revs. Be careful because carb cleaner is highly flammable. Keep the spray well away from ignition components and wiring in general.

A more primal method is to put one end of some tubing against, not into, your ear and move the other end around the suspect hoses. You might be able to discern the whistle as air is sucked in through the leak point.

What's the economy gauge reading while idling in D at operating temperaure? Better still, what does a diagnostic vacuum gauge read?

Sixto
95 S420
87 300SDL