I'm sorry, I should have mentioned that I had already localized that hissing sound. I managed to alleviate it temporarily half a year ago. It's coming from a round like instrument with a small cylinder on top of it, having about the width of a quarter. It's under the air intake manifold, next to the firewall, close to the throttle lever and spring. There's a small piston in it, I think. It's cover was missing. So I made a cover. I put a penny on it and sealed it with bubble gum. It worked and the hissing quit. I'm not sure though if the heating was malfunctioning at this time. It was winter you know. Now it is hissing again. So if anyone thinks this could be causing my heat problem I'll try to seal it again. The noise doesn't really bother me.
Thanks

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1978 350SLC
250000km

[This message has been edited by Mark Sare (edited 09-29-2000).]