Check timing belt tension. Timing belts can harmonically flutter on the long span after the crank pulley on the exhaust side, make noise and even touch the cover. Belt length affecting tension and belt flexibility will vary by temp, which would change the flutter. The DOHC B234F (for example) has a small stainless steel wear plate on the inside of the cover at the mid-span point for that reason. If you've got an automatic hydraulic tensioner then it may be slowly failing (I didn't check, but I'm guessing you do, especially if it's an interference engine). Inspect carefully at the base of the pulley arm to see if there's any hint oil residue indicating a leak. Other than measuring belt tension. there is no good test of hydraulic tensioners that have only started to go. They are considered a wear item and need occasional replacement.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now