Controlling the variations of sound effects using neural audio synthesis models has been a challenging task. Differentiable digital signal processing (DDSP) provides a lightweight solution that achieves high-quality sound synthesis while enabling deterministic acoustic attribute control by incorporating pre-processed audio features and digital synthesizers. In this research, we introduce DDSP-SFX, a model based on the DDSP architecture capable of synthesizing high-quality sound effects while enabling users to control the timbre variations easily. We integrate a transient modelling algorithm in DDSP that achieves higher objective evaluation scores and subjective ratings over impulsive signals (footsteps, gunshots). We propose a novel method that achieves frame-level timbre variation control while also allowing deterministic attribute control. We further qualitatively show the timbre transfer performance using voice as the guiding sound.