Hello Jerry and welcome to the forum. The motorboat osc generally occurs in audio stages due to dried out electrolytic decoupling cap. Ur problem sounds (ugh!) like something further toward the rx front end. U might try bridging decoupling caps thruout to see if it quits. an interesting problem
I have also seen this in old rx with converter tubes like 6K7 etc. again try more decoupling caps .. gl
But if in audio stages, why is this happening only at one spot across the 0-200kc PTO spread (2.5 - 2.7MHz) and no other frequency? I checked on voltages and resistances and all good. I think it has to be after the RF and before Audio so IF amps?? There is one old electrolytic between plate and cathode in the Prod Det which follows the IF amps.. I will replace that and if that doesn't work I will try to bridge key 0.01uF decoupling caps in the IF Gain amp circuits.
Thanks for any advice