Classic/Antique Car Repair: 69 Mustang, convertible mustang, station wagons

Question
I have a 69' Convertible Mustang, 1st Car purchased (17yrs ago). I have done minor resto on her 3 times. A problem that I have noticed that occurs when I mash the gas I get a clanging fluttery sound like the rocker arms are loose. When it is in park and i rev the gas she purs like a raped ape. No clatter. Idles smooth and purfect!!! so what gives?  Question # 2 is she is a convertible, With the top up and the windows up the pass compartment is clear and pleasant, When you drop the windows a vacuume occurs and an breeze of exhaust smell comes from the side quaters where the hydraulic cylinders are on both sides. I have looked for any wholes and i have also boxed in the trunk. there are no vents on the rear quarter pillars. Any ideas?  

Answer
If I understand you correctly, you are saying that this clattering only occurs if you are in drive and putting a load on the engine.  If this is right, the only difference between doing this when you are in park and when you are in drive is that the engine torque will move the engine somewhat when you are in drive, so I suspect you have a bad motor mount, probably on the driver's side, as this is usually the first one to fail.

You didn't tell me which engine you have, but if it is a 6, you can probably grab the valve cover and rock the engine side to side to see if there is any play in the motor mounts - if it is an 8, you'll probalby have to get it up on a lift and pry between the block and the frame to see if you can visually see a crack or other sign of failure in the mount.   If you do, it's a good idea to replace all 3 of them while you are at it.

As for the exhaust coming in, this is hard to guess at, but my first thing to look at after verifying that there are no holes in the underside of the car, is to see how the tail pipe tips are shaped, and where they end.  Station wagons have this problem all the time with fumes coming in the back window, and the only cure is to put in tail pipes that exit to the sides, behind the rear wheels.  You can do that too, but with a convertible you may not want to for looks reasons.  But, your tail pipe(s) should extend at least a couple of inches beyond the rear bumper, and the tip of the pipe should be formed so that it directs the fumes down and to the rear - not straight out the back.

I hope this is some help,

Dick