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Early on I suspected low oil pressure since the car gauge always showed a tad below 40 psi. So I replaced the sender with a mechanical pressure gauge and drove it at about 45 mph in 3rd gear ( out of overdrive) and the pressure was pretty close to 50 psi as advertised for the 356, so I don't think it's an overall oil pressure issue. This is something I just noticed the past month or so. I'm not 100% sure it's lifter/valve noise, I thought it might be exhaust or ignition (pinging or knock) but I replaced the vacuum advance and checked the most obvious exhaust connections and don't see any signs of leakage. It really sounds like it's coming from the front of the engine and I removed the forward valve cover and turned the engine over several revs by hand (by grabbing the fan) and watched the lifters move and tried to get a feeler gauge between the lifter and the valve stem and couldn't. they all seemed pretty tight. I then sprayed some KROIL into each of the front 8 lifters. In addition I've added a quart of Marvel to the oil. Nothing I've done seems to help. I drive the car almost daily and during the first drive of the day I can really hear the clatter accelerating in second and third. Then, as I drive for 15 or 20 minutes I have a harder time hearing it. Now understandably it's harder to hear the noise in traffic, but I've driven around town, and then returned to my neighborhood street where it's relatively quiet and I can barely hear the sound. Needless to say, it's driving me nuts.

You will not be able to get a feeler gauge between lifter and valve as the lifters are spring loaded upward.

With the noise quieting with increasing temperature I would rather suspect piston slap or a wrist pin knock. Test for that by shorting cylinders one at a time while the noise is present and listen for a change.

You also might want to lay a stethescope on the fuel pump.

I'm with Ross on this, as I questioned a few posts ago the symptoms given in Post #1 don't seem to correspond to an oil pressure/hydraulic lifter problem.

Thanks to everybody who replied to this. Well, maybe it is piston slap. I've had the car for a little over a year and like I mentioned, it's just in the last month or so that I've notice the noise, I guess the cylinder could have worn funny in that time.

Does the noise seem to be at engine speed, or about one-half slower? Engine speed would indicate something other than the valves. JWL

It's hard to tell, but I would guess it's at engine speed. As I mentioned, it's most prominent during acceleration on the road, if you rev the engine while stationary you don't hear it, so it has to be under a load I guess. Lately I notice that the engine is running rough at very high revs ( gas to the floor in second gear for example). I'm going to run a compression check on it tomorrow and I'm also replacing the fuel pump this weekend to see if that helps but I'm not too hopeful since the roughness doesn't go away when I engage the electric fuel pump. The car has really run flawlessly for the past year, and I drive it almost on a daily basis since I retired in April. It's just in the last month or so that I've encountered the noise and now the roughness at high revs.

You can about rule out the lifter as they only make noise at low speeds unless they are utterly ruined in which case there is no scenario where they will be quiet.