I have the EXACT same noise comming from my 87 N/A Surpa, in the exact same place and everything. I have been able to localize the noise as happening exactly when the cams line up w/ the timing marks, and further more, the noise is a lot louder on the intake cam rather than the exhaust cam.
I'll agree with the broken valve spring theory, unfortantly, I haven't had the time to confirm this yet. CRE is correct on a way to replace the valve springs with the head on the car, however if you have access to compressed air, I have always put the piston of that cylinder on TDC of the compression stroke and put about 120 psi of pressure in that cylinder and thats enough to keep the valves closed while you replace the spring. You will need the "special tool" required to compress the spring while pulling the locks on the retainers, unless your extremely strong and fast at being able to push that valve spring down and pulling them. Good luck putting them back on w/ the latter method. In short, buy the tool.
In the next few weeks, I might pull my cams out and see which one is bad for the sake of doing it, but b/c I also have a hell of a rod knock, I'm probably just going to replace my engine w/ a 7M-GTE motor instead. Good luck though and I'll stay tuned to see what you come up with. If I happen to find the time to pull my cams out, I'll try to post pics for you.
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