G'day

A rattling or sizzling sound at mid range or above can be a symptom of pre-ignition ie fuel igniting too soon in the power stroke. It can be caused by a plug which is too hot, by too much nitro (in a high compression engine) and/or a lean mixture. It will result in damage to the engine if not corrected. An over large prop will also put extra load on the engine and can cause this effect.

I have seen one particularly bad case of this with an ASP 120 two stroke in an Ultimate Bi-Plane. The tank was very tall and narrow and if the engine were tuned at the top of the tank it would be running lean by about half way down the tank and would stop well before the bottom of the tank. If tuned at the bottom of the tank, it was way too rich when the tank was full. The solution was to fit a fuel pressure regulator such as a Cline or to use a second clunk for the vent line. This is called a uniflow tank setup. If you do a search here you are bound to find into on how to set it up.

Your Thunder Tiger engine is a high quality engine and if it just needs some bearings, they are easily replaced and will give the engine a new lease of life.