Noisy until warmed up Options

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Lately I have noticed a rattling noise until my engine is warmed up. It almost sounds like plastic vibrating somewhere. It sounds like it may be coming from the passenger side of the engine compartment. Maybe under the timing belt cover. Do timing belts get noisy when they start to go, or maybe the tensioner? I was told that the timing belt had been changed at 200k but that was before I purchased it. I noticed on the cover it has a sticker saying it was changed at 104k and I have 244k on it now so I will probably just go ahead and change it to be on the safe side. But anyways, are there any common problems on the Celica's that could cause a rattling sound until it warms up? It is mostly at idle and slow speeds(parking lot) but then as soon as I get on the gas it will start to get louder for just a split second and then go away. Once I come to a stop, and let the engine come back to idle, it will typically start rattling again until it has been running for a few minutes.

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