Originally Posted by MrBill66Malibu
Radio bracket or dash support bracket
I thought about that, but it seems too specific to the speed of the car.
You might check your bellhousing bolts.
Hadn't really thought about that one. Doesn't seem like that would cause the noise that I'm hearing, but I've seen stranger things in the past.
This may sound stupid, but based on your description, it almost sounds like it's in detonation (pinging). What compression is the motor/how much timing/what kind of fuel are you running?
I've had engine detonation on other vehicles, and this is nothing like that at all. Detonation should stop immediately after easing off of the throttle (in my experience), but this lingers for a bit and slowly fades away. Other than a mild cam and camel humps the engine hasn't really been worked over. I would have to say that the compression is low, don't know where the timing is at currently, and always run 91-93 octane from the pump.
HEI distributor hitting firewall?
Bad engine mounts?
Doesn't have an HEI dist, so clearance isn't a problem. There don't seem to be any other indicators that it might be a motor mount, but I'll add it to the checklist and look into as I go.
I am thinking that it does have something to do with the speedo cable. I was going to disconnect it at the trans last night and give it a test drive, but the rain never let up. If the noise stops after the cable has been removed from the trans, at least I'll know that it is the cable itself or something inside the speedometer.
I do appreciate the input folks, it gives me a number of different avenues of approach. I can't imagine that it'll take me too long to track this down once I set my sights on it (didn't really give it a lot of thought since it was supposed to be the TC).