My 1987 300SDL has only 52k miles, bought it about two months ago. When driving down the road with either the heater on (I am from the north - we have two seasons - Christmas and winter!) or the A/C either - I hear a hissing sound from time to time. NOT all the time, not real loud.

I suspect I have a vacuum leak. Ideas? Replace all the vacuum lines since the car is 14 years old?

Can this be done by a DIY person or not?

Thanks.